Water Quality in the Delmarva Peninsula 

Delaware, Maryland, and Virginia, 1999–2001

U.S. Department of the Interior

U.S. Geological Survey

Circular 1228
Points of Contact and Additional Information

The companion Web site for NAWQA summary reports:

http://water.usgs.gov/nawqa/nawqa_sumr.html/

Other NAWQA summary reports

River Basin Assessments

Acadian-Pontchartrain Drainages (Circular 1232)

Albemarle-Pamlico Drainage Basin (Circular 1157)

Allegheny and Monongahela River Basins (Circular 1202)

Apalachicola-Chattahoochee-Flint River Basin (Circular 1164)

Central Arizona Basins (Circular 1213)

Central Columbia Plateau (Circular 1144)

Central Nebraska Basins (Circular 1163)

Connecticut, Housatonic and Thames River Basins (Circular 1155)

Cook Inlet Basin (Circular 1240)

Delaware River Basin (Circular 1227)

Eastern Iowa Basins (Circular 1210)

Georgia-Florida Coastal Plain (Circular 1151)

Great and Little Miami River Basins (Circular 1229) 

Great Salt Lake Basins (Circular 1236)

Hudson River Basin (Circular 1165)

Island of Oahu (Circular 1239)

Kanawha - New River Basins (Circular 1204)

Lake Erie - Lake Saint Clair Drainages (Circular 1203)

Long Island - New Jersey Coastal Drainages (Circular 1201)

Lower Illinois River Basin (Circular 1209)

Lower Susquehanna River Basin (Circular 1168)

Lower Tennessee River Basin (Circular 1233)

Las Vegas Valley Area and the Carson and Truckee River Basins (Circular 1170)

Mississippi Embayment (Circular 1208)

Mobile River Basin (Circular 1231)

New England Coastal Basins (Circular 1226)

Northern Rockies Intermontane Basins (Circular 1235)

Ozark Plateaus (Circular 1158)

Potomac River Basin (Circular 1166)

Puget Sound Basin (Circular 1216)

Red River of the North Basin (Circular 1169)

Rio Grande Valley (Circular 1162)

Sacramento River Basin (Circular 1215)

San Joaquin-Tulare Basins (Circular 1159)

Santa Ana Basin (Circular 1238)

Santee River Basin and Coastal Drainages (Circular 1206)

South-Central Texas (Circular 1212)

South Platte River Basin (Circular 1167)

Southern Florida (Circular 1207)

Trinity River Basin (Circular 1171)

Upper Colorado River Basin (Circular 1214)

Upper Illinois River Basin (Circular 1230)

Upper Mississippi River Basin (Circular 1211)

Upper Snake River Basin (Circular 1160)

Upper Tennessee River Basin (Circular 1205)

Western Lake Michigan Drainages (Circular 1156)

White River Basin (Circular 1150)

Willamette Basin (Circular 1161)

Yakima River Basin (Circular 1237)

Yellowstone River Basin (Circular 1234)

National Assessments

The Quality of Our Nation‘s Waters—Nutrients and Pesticides (Circular 1225)

Delmarva Peninsula contact and Web site:

USGS State Representative

U.S. Geological Survey

Water Resources Discipline

1289 McD Drive

Dover, DE 19901

e-mail: jmdenver@usgs.gov

http://md.water.usgs.gov/Delmarva

National NAWQA Program:

Chief, NAWQA Program

U.S. Geological Survey

Water Resources Discipline

12201 Sunrise Valley Drive, M.S. 413

Reston, VA 20192

http://water.usgs.gov/nawqa/

Front cover: Soybeans growing on well-drained farmland in New Castle County, Delaware (photograph by 

David Usher, U.S. Geological Survey).

Back cover: Left, collecting a ground-water sample for radon analysis (photograph by David Hudson, 

U.S. Geological Survey); center, collecting a ground-water sample for age-dating (photograph by 

Deborah Bringman, U.S. Geological Survey); right, documenting land use at a ground-water site 

(photograph by Deborah Bringman, U.S. Geological Survey). 
U.S. Department of the Interior

U.S. Geological Survey

Water Quality in the Delmarva Peninsula,

Delaware, Maryland, and Virginia,

1999–2001

By Judith M. Denver, Scott W. Ator, Linda M. Debrewer, Matthew J. Ferrari,

Jeffrey R. Barbaro, Tracy C. Hancock, Michael J. Brayton, and Mark R. Nardi

Circular 1228
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Denver, Judith M., Ator, Scott W., Debrewer, Linda M., Ferrari, Matthew J., Barbaro, Jeffery R., Hancock, Tracy C., 

Brayton, Michael J., and Nardi, R., Mark, 2004, Water quality in the Delmarva Peninsula, Delaware, Maryland, and 

Virginia, 1999–2001: Reston, Va., U.S. Geological Survey Circular 1228, 36 p.

Library of Congress Cataloging-in-Publication Data

Water quality in the Delamarva Peninsula, Delaware, Maryland, and Virginia, 1999–2001 / 

 by Judith M. Denver ... [et. al.].

 p. cm. -- (Circular ; 1228)

 Includes bibliographical references.

 ISBN 0-607-92635-X

 1. Water quality -- Delmarva Peninsula. 2. Agricultural chemicals -- Environmental 

 aspects -- Delmarva Peninsula. I. Denver, J. M. II. Geological Survey (U.S.) 

 III. U.S. Geological Survey circular ; 1228.

 TD224.D4W38 2003

 363.739’42’0975--dc22 2003063066
Contents

National Water-Quality Assessment Program ......................................................... iv

What kind of water-quality information does the NAWQA Program provide? ..................... v

Introduction to this Report .............................................................................. vi

Summary of Major Findings .............................................................................. 1

Stream and River Highlights ..................................................................... 1

Ground-Water Highlights ........................................................................... 2

Introduction to the Delmarva Peninsula ............................................................... 3

Land Use ............................................................................................. 3

Hydrogeology and Water Use ..................................................................... 3

Ground-Water Quality .............................................................................. 4

Surface-Water Quality .............................................................................. 5

Major Findings ............................................................................................. 6

Nutrients in Ground Water and Streams ......................................................... 6

Nitrate is widespread in the surficial aquifer, including deeper parts used 

for drinking water .................................................................. 6

Nutrient concentrations in streams reflect human sources ........................... 9

NATIONAL PERSPECTIVE—Nutrient concentrations on the Delmarva Peninsula

are among the Nation’s highest. ......................................................... 10

Pesticides in Ground Water and Streams ...................................................... 12

Herbicides are frequently detected in the surficial aquifer ........................... 12

What is a pesticide degradate? ............................................................ 14

Pesticides are commonly detected in streams .......................................... 16

Delmarva Peninsula NAWQA study contributes to understanding of water

quality in the Chesapeake Bay ................................................... 17

NATIONAL PERSPECTIVE—Herbicide concentrations in streams and ground

water in agricultural areas on the Delmarva Peninsula are similar to 

those in other intensively farmed areas where corn and soybeans 

are grown. ................................................................................. 18

Other Ground-Water Quality Issues ............................................................ 19

Volatile organic compounds are detected in ground water ........................... 19

Radon occurs in water throughout the surficial aquifer ................................. 19

Dissolved iron is present in ground water from poorly drained areas .................. 20

Arsenic is common in ground water at low concentrations ........................... 20

Reconnaissance for Arsenic in a Poultry-Dominated Basin on the Delmarva

Peninsula .............................................................................. 21

Study Unit Design ....................................................................................... 22

References Cited .......................................................................................... 24

Glossary ................................................................................................... 26

Appendix—Water-Quality Data from the Delmarva Peninsula in a National Context ............ 27
National Water-Quality Assessment Program

The quality of the Nation’s water resources is integrally linked to the long-term availability of water 

that is clean and safe for drinking and recreation and also suitable for industry, irrigation, and habitat 

for fish and wildlife. Recognizing the need for long-term, nationwide assessments of water resources, 

the U.S. Congress has appropriated funds since 1991 for the USGS to conduct the National WaterQuality Assessment (NAWQA) Program. Scientists in the NAWQA Program work with partners in 

government, research, and public-interest groups to assess the spatial extent of water-quality conditions, the way water quality changes with time, and the effects of human activities and natural factors 

on water quality. This information is useful for guiding water-management and protection strategies, 

research, and monitoring in different hydrologic and land-use settings across the Nation.

In 1991, NAWQA began evaluating the quality of streams, ground water, and aquatic ecosystems in 

more than 50 major river basins and aquifer systems across the Nation, referred to as “Study Units.” 

As indicated on the map, timing of the assessments varies within the Program’s rotational design: 

about one-third of all Study Units are intensively investigated for 3 to 4 years, with trends assessed 

every 10 years. 

In 2001, the NAWQA Program entered its second decade of investigations, and an intensive reassessment of water conditions was begun to determine trends, based on 10 years of comparable monitoring 

data collected at many of the streams and ground-water sites. The next 10 years of study also will fill 

critical gaps in characterizing water-quality conditions and increase understanding of processes that 

control water-quality conditions, which will better establish critical links among sources of contaminants, their transport through the hydrologic system, and the potential effects of contaminants on 

ecological health and on the quality of drinking water.

The Delmarva Peninsula assessment is one of two special studies activated in 1999 for the purpose 

of piloting study techniques for use in NAWQA’s second decade of investigations. Specifically, the 

Delmarva Peninsula assessment piloted techniques for (1) monitoring trends in ground-water quality, 

(2) evaluating transport of agricultural chemicals to streams, and (3) assessing the effects of agricultural chemicals from corn, soybean, and small grain production on shallow ground water and surface 

water. The Delmarva Peninsula assessment builds upon monitoring data that the NAWQA Program 

collected previously in the peninsula from 1987 through 1991, as part of pilot studies conducted before 

full Program implementation in 1991. These data provided a baseline characterization of pesticides, 

nutrients, and trace elements in ground water and at surface water during base flow.

1991–95 

1994–98

1997–2001

Not yet scheduled

High Plains Regional

Ground Water Study, 

1999–2004

NAWQA Study Units— 

Assessment schedule

Delmarva Peninsula

iv
What kind of water-quality information does the NAWQA 

Program provide?

Water-quality assessments by a single program cannot possibly address all of the Nation’s 

water-resources needs and issues. Therefore, it is necessary to define the context within which 

NAWQA information is most useful. 

• Total resource assessment—NAWQA assessments are long-term and interdisciplinary, 

and include information on water chemistry, hydrology, land use, stream habitat, and 

aquatic life. Assessments are not limited to a specific geographic area or water-resource 

problem at a specific time. Therefore, the findings describe the general health of the total 

water resource, as well as emerging water issues, thereby helping managers and decision 

makers to set priorities.

• Source-water characterization—Assessments focus on the quality of the available, 

untreated resource and thereby complement (rather than duplicate) Federal, State, and 

local programs that monitor drinking water. Findings are compared to drinking-water 

standards and health advisories as a way to characterize the resource.

• Compounds studied—Assessments focus on chemical compounds that have wellestablished methods of investigation. It is not financially or technically feasible to assess 

all the contaminants in our Nation’s waters. In general, the NAWQA Program investigates 

those pesticides, nutrients, volatile organic compounds, and metals that have been or are 

currently used commonly in agricultural and urban areas across the Nation. A complete 

list of compounds studied is on the NAWQA Web site at water.usgs.gov/nawqa.

• Detection relative to risk—Compounds are measured at very low concentrations, often 

10 to 100 times lower than Federal or State standards and health advisories. Detection of 

compounds, therefore, does not necessarily translate to risks to human health or aquatic 

life. However, these analyses are useful for identifying and evaluating emerging issues, 

as well as for tracking contaminant levels over time.

• Multiple scales—Assessments are guided by a nationally consistent study design and 

uniform methods of sampling and analysis. Findings thereby pertain not only to water 

quality of a particular stream or aquifer, but also contribute to the larger picture of how 

and why water quality varies regionally and nationally. This consistent, multiscale 

approach helps to determine if a water-quality issue is isolated or pervasive. It also 

allows direct comparisons of how human activities and natural processes affect water 

quality in the Nation’s diverse environmental settings. 

v
vi

Introduction to this Report

This report contains the major findings of a 1999–2001 assessment of water quality in ground 

water and streams in the Delmarva Peninsula. It is one of a series of reports by the NAWQA 

Program that present major findings on water resources in 51 major river basins and aquifer 

systems across the Nation. 

In these reports, water quality is assessed at many scales—from local ground-water flow paths 

to regional ground-water networks and in surface water—and is discussed in terms of local, 

State, and regional issues. Conditions in the Delmarva Peninsula are compared to conditions 

found elsewhere and to selected national benchmarks, such as those for drinking-water quality 

and the protection of aquatic organisms. 

This report is intended for individuals working with water-resource issues in Federal, State, or 

local agencies, universities, public-interest groups, or the private sector. The information will 

be useful in addressing a number of current issues, such as source-water protection, pesticide 

registration, human health, drinking water, hypoxia and excessive growth of algae and plants, 

the effects of agricultural land use on water quality, and monitoring and sampling strategies. 

This report also is for individuals who wish to know more about the quality of water resources 

in areas near where they live, and how that water quality compares to other areas across the 

Nation.

Other products describing water-quality conditions in the Delmarva Peninsula are available. 

Detailed technical information, data and analyses, methodology, models, graphs, and maps 

that support the findings presented in this report can be accessed from http://md.water.usgs.gov/

delmarva. Other reports in this series and data collected from other basins can be accessed from 

the national NAWQA Web site (http://water.usgs.gov/nawqa).

“The Delmarva NAWQA 

study provides us with a 

timely report and assessment 

of water-quality trends for 

nutrients and pesticides in our 

ground-water and surfacewater resources and reminds 

us all that much more needs 

to be done to protect and 

improve the water resources 

of Delaware.”

Kevin C. Donnelly

Director, Division of Water Resorces, 

Delaware Department of Natural 

Resources and Environmental Control
Summary of Major Findings

1

Stream and River Highlights

The concentrations of nitrogen, phosphorus, and pesticides in streams and rivers of the Delmarva Peninsula reflect 

the predominance of agriculture and the soil and aquifer 

conditions that promote transport of agricultural chemicals 

to streams. Other sources of nutrients and pesticides include 

septic systems, sewage, and urban and suburban chemical 

applications. Ground water discharged from the surficial 

aquifer is a primary source of water and nitrate in streams. 

Pesticides and phosphorus are contributed to streams primarily in runoff during storms; soluble pesticides also move to 

streams through ground water. Contaminated streams and 

rivers affect downstream estuarine waters by contributing 

excessive nutrients that can lead to eutrophication (blooms 

of algae and other nuisance plants) and hypoxia (areas of 

low dissolved oxygen), resulting in fishkills and loss of other 

living resources. Concentrations of nitrogen, phosphorus, 

and herbicide compounds in stream waters of the Delmarva 

Peninsula are similar to those in other intensively agricultural 

areas of the Nation.

• Nitrate concentrations were greater than 3 mg/L 

(milligrams per liter as nitrogen) in about one-half of 

the headwater streams during base flow in the spring. 

Nitrate concentrations in streams generally increased 

with increasing agricultural intensity in upstream 

watersheds, particularly in areas with permeable, 

well-drained, and well-oxygenated sediments (p. 9).

• Phosphorus is transported to streams and rivers primarily with sediment in overland runoff during storms. 

Concentrations of phosphorus were typically less than 

0.1 mg/L at base flow in well-drained areas but were 

as high as 3 or 4 mg/L at base flow in poorly drained 

areas (p. 11).

• Pesticides, primarily herbicides, are detectable throughout the year in streams of the peninsula, commonly 

occurring in mixtures of multiple compounds. At 

least five different compounds were detected in water 

from each of 23 streams sampled during base flow in 

the spring. Concentrations in streams generally were 

higher than in ground water (p. 16).

• Concentrations of herbicides are highest in the spring, 

after agricultural applications. Instream concentrations 

of individual compounds rarely exceeded concentrations known to be harmful to aquatic plants and animals, 

although such concentrations have not been established 

for many compounds (p. 16).

• The most commonly detected pesticides in streams were 

the herbicides used on corn, soybeans, and small grain 

crops, including atrazine, metolachlor, and alachlor. 

These also are among the most heavily used herbicides 

on the peninsula. Non-crop pesticides also were detected, 

but less frequently. These include the herbicides prometon (a weed killer used along rights-of-way) and pronamide (commonly used on ornamental shrubs) (p. 16).

Delaware Bay

t At an l ic Ocean

Ches

a

ape ke Bay

Major drainage 

divide

MILES

KILOMETERS

National Elevation Dataset, 1999.

Chesapeake Bay

FALL LINE

ATLANTIC OCEAN

80

40

75

Location of Delmarva Peninsula

38

80 ̊ 75 ̊

40 ̊

38 ̊

Dover, Del.

Salisbury, Md.

Wilmington, Del.

Highest, 497

Lowest, 0

Digital shaded-relief map

Value in feet

Pa.

Md.

Del.

Va.

W. Va. D.C.

N.J.

Chester River

Ch a op t nk River

Nanticoke Rive r

Pocomoke River

The Delmarva Peninsula covers about 6,000 square miles and 

includes most of Delaware and parts of Maryland and Virginia 

east of the Chesapeake Bay. Located entirely in the Coastal 

Plain Physiographic Province, it has a broad central upland 

with flat to gently rolling topography, flanked by low plains that 

slope toward surrounding water bodies. Two major estuaries 

and the Atlantic Ocean surround the peninsula.

Major influences on water quality in streams and rivers

• Agriculture in watersheds

• Ground-water discharge of nitrate and pesticides

• Overland runoff of phosphorus, sediment, and 

pesticides during storms
2 Water Quality in the Delmarva Peninsula, Delaware, Maryland, and Virginia, 1999–2001

Ground-Water Highlights

The surficial aquifer that blankets the Delmarva 

Peninsula is particularly susceptible to contamination from 

human activities because soils and aquifer sediments are relatively permeable and the water table is shallow. This aquifer is 

an important source of drinking water, recharge to underlying 

confined aquifers, and base flow to streams. Nearly half of 

the peninsula is used for agriculture, and chemicals applied 

for crop production are commonly detected in ground water. 

Concentrations of nitrate and herbicide compounds in ground 

water of the Delmarva Peninsula are among the highest in the 

Nation. Other chemicals associated with urban and suburban 

land uses, including compounds from fuels and solvents, also 

are detectable in ground water in those areas. 

• Nitrate is widespread in the surficial aquifer, including 

parts of the aquifer used for drinking water. Concentrations of nitrate exceeded 10 mg/L, the Federal standard 

for drinking water, in about one-third of 29 samples 

from wells in the part of the aquifer used for domestic 

water supply (median well depth 45 feet); the median 

concentration was 5.5 mg/L. In 30 samples from deeper 

parts of the aquifer used for public water supply in 

Delaware (median well depth 80 feet), concentrations 

were similar (median 5.2 mg/L), although concentrations exceeded 10 mg/L in only one sample (p. 6).

• Concentrations of nitrate in ground water are highest beneath areas with well-drained, permeable soils, 

where dissolved-oxygen concentration also is high 

(greater than 1 mg/L) and lowest beneath areas with 

poorly drained soils, where dissolved-oxygen concentration is low (less than 1 mg/L), regardless of 

overlying land use (p. 7–8).

• Decreases in nitrate with depth in the surficial aquifer reflect changes in nitrogen fertilizer use over time 

rather than losses from degradation of nitrate to other 

chemical forms. Deeper water is commonly older, and 

nitrate concentrations reflect lower historical rates of 

fertilizer application. Nitrate is stable in oxygenated 

ground water and can persist along ground-water flow 

paths for decades (p. 7).

• Between 1988 and 2001, nitrate concentrations 

increased an average of about 2 mg/L in parts of 

the surficial aquifer used for domestic water supply 

(median well depth 45 feet) but were unchanged in 

shallower ground water beneath agricultural areas 

(median well depth 22 feet) (p. 9).

• Pesticides are widespread in the surficial aquifer 

underlying the Delmarva Peninsula, typically at concentrations less than 1 μg/L (microgram per liter). 

Commonly used herbicides (metolachlor, atrazine, and 

alachlor and their degradates) were most frequently 

detected; insecticides and fungicides were rarely 

detected (p. 12).

• A wide variety of pesticides and their degradates 

were present in water from the surficial aquifer used 

for drinking in urban and agricultural areas. In total, 

19 different compounds were detected in water from 

public-supply wells, and 27 different compounds were 

detected in water from the part of the surficial aquifer 

used for domestic supply (p. 13).

• Volatile organic compounds (VOCs) are common in 

the surficial aquifer. Measured concentrations were 

typically below 1 μg/L (microgram per liter), and none 

were higher than Federal standards for the protection of 

drinking water where those standards exist. VOCs were 

much less frequently detected in domestic wells in 

rural areas than in public-supply wells in more populated areas (p. 19).

 Major influences on ground-water quality

• Agricultural, suburban, and urban land-use 

activities in well-drained areas with permeable 

sediments

• Sandy sediments and dissolved oxygen in the surficial aquifer, which promote the transport of nitrate 

and pesticides in ground water

Small

agricultural

streams

Shallow

agricultural

ground water

Surficialaquifer

wells 1

HERBICIDES

INSECTICIDES

NITRATE

PHOSPHORUS 

RADON

TRACE ELEMENTS 2

ARSENIC

VOLATILE ORGANICS 3

— Not assessed

—

—

—

— —

2 Mercury and other metals.

3 Solvents, refrigerants, fumigants, and gasoline products.

— —

1 Includes domestic supply wells and deeper monitoring wells.

 Selected Indicators of Water Quality

Proportion of samples with detected concentrations

greater than or equal to health-related National guidelines 

for drinking water, protection of aquatic life, or the desired

goal for preventing nuisance plant growth

Proportion of samples with detected concentrations 

less than health-related national guidelines for drinking 

water, protection of aquatic life, or below the desired goal 

for preventing nuisance plant growth

Proportion of samples with no detections
Introduction to the Delmarva Peninsula

3

The Delmarva Peninsula, a primarily 

rural lowland along the Atlantic Coast in 

the Eastern United States, contains most 

of Delaware and parts of Maryland and 

Virginia (fig. 1). The climate is humid 

and subtropical, with an average annual 

rainfall of 44 inches. The entire peninsula lies within the Coastal Plain 

Physiographic Province, which generally 

is a flat, seaward-sloping lowland with 

areas of moderate topographic relief. In 

2000, about 1.1 million people lived on 

the peninsula; the population is increasing rapidly in the urban area south of 

Wilmington, Del., and near resorts along 

the Atlantic coastline.

Land Use 

Agriculture is the dominant land 

use on the peninsula, covering about 

48 percent of the land (fig. 1). Although 

plant nurseries and sod and dairy farms 

are common in some areas, agricultural land is used primarily to grow 

corn and soybeans for poultry feed. 

More than 600 million broiler chickens were produced on the peninsula 

in 2000 (Delmarva Poultry Industry, 

Inc., 2002). Other crops include small 

grains, potatoes, vegetables, and fruit. 

Woodlands, commonly interspersed 

with agricultural areas, make up about 

one-third of the land area. Wetlands

are common in riparian and coastal 

zones and in inland forested areas and 

depressions. Only about 7 percent of the 

peninsula is classified as urban, primarily 

south of Wilmington, Del. (population of 

about 350,000), and in small towns. The 

largest cities are Dover, Del. (population 

of about 32,000), and Salisbury, Md. 

(population of about 24,000).

Hydrogeology and Water Use

A wedge of sand, gravel, silt, 

clay, and shells underlies the Delmarva 

Peninsula. These sediments thicken 

from 0 ft (feet) to the north to more than 

8,000 ft along the Atlantic Coast 

Figure 1. The Delmarva Peninsula is primarily rural, with small towns and residences interspersed among agricultural and 

forested land. Major areas of urban growth are to the north and along the Atlantic coastline.

c
4 Water Quality in the Delmarva Peninsula, Delaware, Maryland, and Virginia, 1999–2001 ESTIMATED WATER USE IN 1995, IN MILLIONS OF GALLONS PER DAY Public supply Domestic supply Commercial and industrial Irrigation

75

50

25

0

Surface water

Ground water

Figure 2. Sediments underlying the Delmarva Peninsula form an alternating series of confined aquifers and associated confining 

units, overlain by an extensive surficial aquifer that is under water-table conditions (modified from Shedlock and others, 1999).

of Maryland and form an alternating 

series of confined aquifers and confining 

units, overlain by an extensive unconfined surficial aquifer (fig. 2). Most 

public water supply comes from either 

the confined aquifers or relatively thick 

parts of the surficial aquifer. The surficial 

aquifer also provides water to individual 

homes and small public-supply systems 

in rural areas (fig. 3). In the northern part 

of the peninsula, rivers of the adjacent 

Piedmont Physiographic Province are 

a major source of public and industrial 

water supply.

The recharge area for the surficial 

aquifer includes most of the land surface of the Delmarva Peninsula. Most 

ground-water recharge occurs during the 

winter and spring, when precipitation and 

infiltration into soils are greatest. Water in 

the surficial aquifer typically flows along 

relatively short flow paths (distances of 

several hundred feet to less than a few 

miles) toward discharge areas in streams 

and estuaries or moves downward as 

recharge to confined aquifers. Water from 

the surficial aquifer typically reaches discharge areas in less than 50 years (Dunkle 

and others, 1993).

Streams on the peninsula generally become tidal within 10 miles of 

their headwaters. The largest nontidal 

watershed area is 113 mi2 (square miles), 

although most streams drain less than 

10 mi2. Streams on the peninsula receive 

most of their flow from ground-water 

discharge (Bachman and others, 1998). 

Streamflow generally is greater in the 

winter and spring, when ground-water 

discharge is greatest and evapotranspitation is least, when compared to summer 

and fall. Streamflow peaks during storms, 

when it is augmented by overland runoff

(fig. 4).

Ground-Water Quality

Land-use activities can contaminate shallow ground water used 

for drinking water. Certain chemicals 

that are applied to the land surface in 

quantities greater than crops, lawns, or 

golf courses can use move easily through 

permeable surficial soils and sediments 

to the water table and can contaminate 

shallow ground water. Other sources 

of ground-water contamination include 

leaking storage tanks, septic systems, and 

chemical spills. Under natural conditions 

(defined as those with no chemical input 

from human activity), water in the surficial aquifer contains chemicals derived 

from rainfall, soils, and aquifer sediments. 

Because soils and sediments in most areas 

of the peninsula do not dissolve easily, 

concentrations of chemical constituents 

occurring naturally in ground water are 

typically only slightly higher than the low 

concentrations in rainfall. Concentrations 

of chemicals from human sources commonly exceed concentrations of those that 

occur naturally in water in the surficial 

aquifer of the Delmarva Peninsula.

Crop and poultry production 

have widespread effects on groundwater quality. Inorganic fertilizers 

contain nitrogen and phosphorus that can 

enter the ground-water system. Poultry 

manure, commonly applied to fields to 

supply nitrogen to crops, contributes 

large amounts of nitrogen and phosphorus and trace amounts of arsenic, 

pharmaceuticals, and other chemicals 

used as feed amendments. Agricultural herbicides also have been frequently detected in ground water on the 

Delmarva Peninsula (Koterba and others, 

1993; Blaier and Baxter, 2000).

Figure 3. Ground water is the major 

source of water used on the Delmarva 

Peninsula and the sole source used for 

rural domestic supply. In the most populated part of the peninsula to the north, 

streams that originate in the adjacent 

Piedmont Physiographic Province also 

are a source of drinking-water supply. 

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Introduction 5

Agricultural management practices intended to reduce the transport 

of chemicals from inorganic fertilizer, manure, and pesticides through 

changes in planting methods, application 

practices, and chemical formulations 

have been implemented over the last 

few decades. Many of the nutrients and 

pesticides currently in the water of the 

surficial aquifer were applied several 

decades ago, before these improvements. The new management practices 

should help to reduce concentrations of 

nutrients and pesticides in ground water 

and their transport to surface water. 

Improvements in water quality may not 

be apparent for years or even decades, 

however, because of the slow movement 

of chemicals through the ground-water 

system.

Urban activities also may affect 

shallow ground-water quality. Individual household and community septic 

systems potentially discharge nutrients, 

bacteria, household chemicals, and 

pharmaceuticals to the surficial aquifer. 

Local concerns also include chemical spills, leaking fuel-storage tanks, 

and use of fertilizers and pesticides on 

lawns, golf courses, and along roadways.

Surface-Water Quality

Streams contain chemicals 

from ground water and overland 

runoff. Ground-water discharge is 

a primary source of nitrate in surface water and also is a source of low 

concentrations of pesticides (Bachman 

and others, 1998; Shedlock and others, 

1999). Nitrogen, phosphorus, pesticides, 

and other chemicals also are transported directly to surface water through 

overland runoff and in effluent from 

sewage-treatment plants and industrial 

discharges.

Hydrologic conditions can 

affect surface-water quality. Water 

quality in streams and rivers varies in response to changes in hydrologic conditions that are affected by 

weather patterns; therefore, information on precipitation and streamflow 

is important for assessing water 

quality (fig. 4). When precipitation 

and runoff are below average, concentrations of chemicals in streams 

are due primarily to the discharge of 

ground water to the streams; concentrations of constituents that attach to soil 

Figure 4. Streamflow varies seasonally on the Delmarva Peninsula, the highest 

flows typically occurring in the winter and spring when the water table is high and 

shallow ground-water discharge to streams is greatest. Lowest flows typically 

occur in the summer and fall, when the water table is lower and there is more 

evapotranspiration. Streamflow is augmented by overland runoff during storms. 

During the study period, streamflow was generally at or below normal through 

1999 and at or above normal during 2000 and 2001.

particles and streambed sediments, 

such as phosphorus and selected organic 

contaminants and trace elements, are 

lower than average. During high flows 

caused by runoff, however, when sediment transport is greatest, concentrations 

of chemicals that attach to sediment 

particles are higher than average. High 

flows caused by runoff also can dilute 

the concentrations of chemicals from 

ground water and effluent discharges.

Degraded water quality in 

streams can affect estuaries that 

are important for recreation and the 

local economy. Excessive nitrogen and 

phosphorus in streams and estuaries 

can cause accelerated growth of aquatic 

plants and algae that block sunlight 

from the water column, a process called 

eutrophication. Eutrophic conditions 

contribute to the loss of submerged 

vegetation, which is needed to provide 

habitat for ecologically and economically important species and to produce 

dissolved oxygen in the water column. 

The excessive algae and plant material 

subsequently decay, reducing dissolved 

oxygen needed by fish and other aquatic 

life; fishkills are not uncommon during eutrophic conditions. In addition to 

excessive nutrients and eutrophication, 

pesticides and other organic contaminants from industrial sources and fuels 

also are of concern because of their 

potential toxic effects on aquatic biota 

(Phillips and Caughron, 1997).

The peninsula provides important 

habitat for wildlife and supports large 

populations of migratory birds. (Photograph by Mark Nardi, U.S. Geological 

Survey.)

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Major Findings

Nutrients in Ground Water 

and Streams

The primary sources of nutrients 

on the Delmarva Peninsula are inorganic fertilizer and manure (accounting 

for more than 95 percent), although 

other sources may be substantial in 

certain areas (fig. 5). Atmospheric 

deposition contributes an additional 

4 percent. Estimated nitrogen contributions from the atmosphere to the peninsula were 15 million pounds in 1997, 

for example (National Atmospheric 

Deposition Program, 2003a,b). Atmospheric deposition may be a particularly 

important source of nutrients for plant 

growth in forested areas, where other 

nutrient sources are limited. Septic systems also contribute nutrients to ground 

water; previous studies on the peninsula, 

however, have found that septic systems 

generally contribute lower concentrations 

of nitrate to shallow ground water than 

do agricultural sources (Denver, 1989; 

Hamilton and others, 1993). Sewagetreatment plants also are nutrient sources 

for streams, but most treatment-plant 

discharges are to tidal waters surrounding 

the peninsula.

Nitrogen inputs from inorganic 

fertilizer have increased considerably 

over the past several decades, whereas 

the input of phosphorus has decreased. 

Nutrient inputs from manure, primarily 

poultry, also have recently decreased, 

although they still contribute substantially to total nutrient input (fig. 6).

Nitrate is widespread in the 

surficial aquifer, including 

deeper parts used for drinking 

water

Concentrations of nitrate are typically above the natural background level 

of 0.4 mg/L (Hamilton and others, 1993) 

in shallow ground water (about 20 to 

25 ft below land surface) beneath farmland. (Although laboratory results were 

reported as “nitrite plus nitrate,” concentrations of nitrite generally were negligible; therefore, these results are reported 

as “nitrate” [as nitrogen] in this report.) 

Specifically, the median concentration 

of nitrate in samples from 29 wells in 

agricultural areas was 5.4 mg/L, and the 

maximum was 37 mg/L (fig. 7). Water in 

about one-third of the wells exceeded the 

Primary Maximum Contaminant Level 

of 10 mg/L (U.S. Environmental 

Protection Agency, 2002b). Nitrate at 

concentrations greater than 10 mg/L 

may cause methemoglobinemia, a 

life-threatening illness in infants. 

Shallow ground water beneath 

farmlands is not commonly used 

for drinking-water supply; however, 

contaminated shallow ground water can 

move downward into the surficial aquifer 

over time and affect the quality of deeper 

ground water that is used for drinking. Elevated concentrations of nitrate 

in deeper parts of the surficial aquifer 

indicate that such movement does occur; 

the distribution of nitrate in the part of 

the aquifer used for domestic supply in 

rural areas (median well depth 45 ft) 

is similar to that in the shallow ground 

water beneath farmland. Specifically, 

median nitrate concentration in the surficial aquifer typically used for domestic 

supply in rural areas (as indicated by data 

from 29 wells, 16 of which were monitoring wells and 13 of which were domestic 

wells) was 5.5 mg/L, and the maximum 

was 27 mg/L. Concentrations of nitrate 

in one-third of the domestic-well samples 

exceeded 10 mg/L. The median nitrate 

concentration in 30 public-supply wells 

in Delaware, located closer to urban areas 

and generally deeper than rural domestic 

Figure 5. Nitrogen sources on the 

Delmarva Peninsula are primarily agricultural. (Data from Brakebill and Preston, 1999.)

NITROGEN SOURCES FOR THE CHESAPEAKE BAY DRAINAGE

 OF THE DELMARVA PENINSULA 

59%

35%

4%

1% 1%

Commercial

fertilizer

Manure

Atmospheric

deposition

Septic systems Point sources 1

1 Includes discharge from sewage-treatment plants.

Figure 6. Nitrogen inputs from inorganic agricultural fertilizer applications 

on the peninsula have increased since the mid–1940s; phosphorus inputs 

have decreased. Manure also is a major source of nitrogen and phosphorus. (Data from Alexander and Smith, 1990; Battaglin and Goolsby, 1994; 

David Lorenz, U.S. Geological Survey, written commun., 2002.)

1940 1950 1960 1970 1980 1990 2000

0

25

50

75

100

125

150

Nitrogen from fertilizer

Nitrogen from manure

Phosphorus from fertilizer

Phosphorus from manure

NITROGEN OR PHOSPHORUS, IN MILLIONS OF POUNDS

These findings are supported by 

the Study Unit Design described on 

pages 22 and 23.

6 
Major Findings 7

wells (median of about 80 ft below land 

surface), also was about 5 mg/L; and the 

maximum concentration was 11 mg/L 

and in only 1 well exceeded the drinkingwater standard. (See “Study Unit Design” 

on pages 22–23 for more information 

about well networks.)

Elevated concentrations of 

nitrate in water from the surficial 

aquifer used for domestic supply are 

of particular concern because many 

homeowners are not aware of possible risks. Unlike public-supply wells, 

domestic wells are not monitored regularly. In addition, many homeowners 

in recently established residential 

areas that rely on domestic wells are 

not aware that chemicals leached from 

previous agricultural or other activities 

can remain in shallow ground water for 

decades.

Median concentrations of nitrate 

are generally highest in water from 

the surficial aquifer where dissolvedoxygen concentrations are greater 

than 1 mg/L (fig. 7). Nitrate is stable 

in the presence of dissolved oxygen. 

Dissolved oxygen typically is present in 

ground water in areas with well-drained 

soils and well-aerated aquifer sediments 

(known as “oxidizing conditions”), 

which are common in the central upland 

area of the peninsula where much of the 

land is agricultural. In areas with poorly 

drained and organic-rich soils, such 

as along coastal lowlands and interior 

wetlands, dissolved-oxygen concentrations are low or zero in the soil zone and 

underlying aquifer (known as “reducing 

conditions”). In these areas, dissolved 

nitrate is converted to nitrogen gas (is 

denitrified) by bacteria that use nitrate 

to obtain energy for growth, so that 

ammonia and organic nitrogen are the 

most common forms of nitrogen in the 

ground water.

Although most poorly drained 

areas on the peninsula are forested, 

some are used for agriculture, such as 

parts of southern Delaware and adjacent 

Maryland. Farmers excavate ditches 

to lower the water table and drain land 

that would otherwise be too wet to 

farm. Nitrate concentrations are typically lower in ground water underlying 

these areas—because of the reducing 

conditions—than in areas with adequate 

natural drainage (Hamilton and others, 

1993).

Concentrations of nitrate are 

lowest in the deepest parts of the 

surficial aquifer, such as in areas used 

for public supply in Delaware (fig. 7, 

fig. 8). Water pumped from the publicsupply wells generally reached the 

aquifer more than 15 years ago (median 

recharge date 1985), when fertilizer 

applications were smaller. In addition, 

large-yield public-supply wells can 

draw water from large areas, which may 

include less agricultural land and more 

areas associated with lower nitrogen 

sources such as forests, wetlands, and 

urban areas. Nitrate concentrations in 

shallow, near-surface ground water in 

agricultural areas (within 30 ft of the 

land surface) generally reflect more 

recent activities on the land (median 

recharge date 1994). 

Nitrate concentrations increase 

in shallow, near-surface ground water 

with increasing amounts of overlying 

agriculture. Nitrate concentrations in 

shallow, near-surface ground water are 

affected by nearby land use when underlying aquifer sediments are oxygenated. Where dissolved oxygen is low or 

absent, nitrate is generally low or absent 

in underlying ground water, regardless 

of overlying land use (fig. 9). Extremely 

high nitrate concentrations in some 

samples may be related to local point 

sources, such as manure piles, rather 

than regional land use.

Nitrate concentrations can 

vary horizontally and vertically 

because of ground-water flow paths 

and land use in recharge areas. Water 

infiltrating through soils and recharging 

the surficial aquifer carries chemicals 

from the land surface in the area where 

the recharge originates (fig. 10). Over 

Figure 7. Concentrations 

of nitrate are typically 

well above the estimated 

natural background 

concentration (0.4 mg/L) 

in the surficial aquifer. 

The median concentration 

of nitrate in oxygenated 

areas exceeds the Federal 

drinking-water standard 

of 10 mg/L in ground water 

used for domestic supply.

 NITRATE, IN MILLIGRAMS PER LITER

DEPTH, IN FEET BELOW LAND SURFACE

0 20 40 60 80 100 120 140

0

10

20

30

40

Federal

drinking-water standard

Ground water used for public supply

Ground water used for domestic supply

Shallow ground water in agricultural areas

Figure 8. Concentrations 

of nitrate commonly exceed 

10 mg/L in the surficial aquifer 

at depths less than 50 ft, 

where many domestic wells 

obtain water. Concentrations are generally less than 

10 mg/L at greater depths, 

where most public-supply 

wells are completed.

Federal

drinking-water

standard

DISSOLVED OXYGEN GREATER THAN 1 mg/L DISSOLVED OXYGEN LESS

THAN 1 mg/L

TOTAL, ALL WELLS

EXPLANATION

Shallow ground water in agricultural areas

Ground water used for domestic supply

Ground water used for public supply 

Number of samples

Maximum

Median

Minimum

23

NITRATE, IN MILLIGRAMS PER LITER 

40

30

20

10

 0

29

30

19

17

23

10

12

7

29
8 Water Quality in the Delmarva Peninsula, Delaware, Maryland, and Virginia, 1999–2001

time, these chemicals are carried along 

ground-water flow paths to deeper parts 

of the ground-water system and eventually discharge to streams. As a result, 

nitrate concentrations in ground water 

beneath a forested area may increase 

with depth if deeper water was derived 

from an upgradient agricultural area; 

likewise, nitrate in water from deep 

wells in urban areas may represent a 

different land use upgradient from the 

well. Because of the variability of land 

use in recharge areas and the presence or 

absence of dissolved oxygen in ground 

water, nitrate concentrations are variable 

at different depths of the surficial aquifer 

underlying areas of different land use.

Nitrate concentrations have 

increased since 1988 in parts of the 

surficial aquifer used for domestic 

supply but did not change in shallow, 

near-surface ground water underlying 

agricultural areas. Nitrate in ground 

water used for domestic supply (median 

well depth 45 ft) in oxygenated parts of 

the surficial aquifer (dissolved oxygen 

greater than 1 mg/L) increased by an 

average of about 2 mg/L, between 1988, 

and 2001, to a median of greater than 

10 mg/L, the Federal drinking-water 

standard (fig. 11). In oxygenated shallow ground water underlying agricultural areas (median well depth 22 ft), 

however, the median nitrate concentration in ground water did not change 

significantly, remaining below 10 mg/L. 

Agricultural management practices have been implemented over the 

last few decades to decrease the movement of nutrients to ground water and 

streams—the success of which may 

help explain stable concentrations in 

the shallow, near-surface ground water. 

Such improvements may not be apparent for years or even decades in deeper 

parts of the surficial aquifer because of 

the relatively slow movement of ground 

water. In poorly oxygenated areas, 

nitrate concentrations were consistently 

low in both the shallow and deep parts 

of the aquifer.

Concentrations of other nutrient species are generally lower than 

concentrations of nitrate in Delmarva 

Peninsula ground water. Concentrations of dissolved phosphorus in ground 

water of the surficial aquifer rarely 

exceeded 0.1 mg/L (see Appendix, 

Figure 9. Nitrate concentrations are typically higher 

in well-oxygenated areas 

(dissolved oxygen greater 

than 1 mg/L) where nitrate is 

stable than in areas where 

dissolved-oxygen concentrations are lower (less than 

1 mg/L). With some exceptions, nitrate concentrations 

increase with increasing 

proportions of overlying 

agriculture.

Figure 10. Patterns in the distribution of agricultural chemicals in the surficial aquifer are determined by land use in aquifer 

recharge areas and ground-water flow paths. Water that contains chemicals from agriculture and other human activities will 

eventually discharge to local streams and rivers. 

AGRICULTURE, IN PERCENTAGE OF AREA

WITHIN 500 METERS OF SAMPLED WELL

Greater than 1 milligram per liter

Less than 1 milligram per liter

Dissolved oxygen

NITRATE, IN MILLIGRAMS PER LITER 

Surficial

aquifer 

Unsaturated

zone

Agriculture Forest Agriculture Riparian zone

Headwater stream

1 to 3 miles

40 to 100 feet

Ground water affected

by agricultural activities

Confining unit

Water table
Major Findings 9

p. 29) despite relatively large applications of phosphorus in inorganic fertilizer and manure (fig. 6). This finding is, 

in large part, explained by the chemical 

properties and mobility of phosphorus. 

Specifically, phosphorus readily attaches 

to soil particles under oxidizing conditions; it does not dissolve and move 

readily to the ground-water system. 

Ground water is therefore typically not 

vulnerable to contamination by phosphorus. Under reducing conditions (where 

dissolved oxygen is absent), however, 

phosphorus does dissolve more readily 

and is present in shallow ground water 

at concentrations greater than 0.1 mg/L. 

Concentrations of ammonia and organic 

nitrogen also are relatively low (less than 

1 mg/L) in the surficial aquifer because 

these forms of nitrogen are readily 

converted to nitrate in well-drained and 

oxygenated soils. Ammonia and organic 

nitrogen are more prevalent in ground 

water under reducing conditions.

Nutrient concentrations 

in streams reflect human 

sources

Throughout the Delmarva 

Peninsula, nitrate concentrations 

in streams were elevated above 

0.6 mg/L, the estimated concentrations 

of nitrate from natural sources and 

atmospheric deposition in streams of 

the Northeastern United States (Clark 

and others, 2000). Nearly all (94 percent) 

of the headwater streams on the peninsula contained nitrate concentrations 

greater than 0.6 mg/L during base flow 

in the spring. About half contained more 

than 3 mg/L. (See “Study Unit Design,” 

p. 22–23, for more information on the 

design of the headwater stream network.) 

Nitrate concentrations in large agricultural streams on the Delmarva Peninsula 

also were often greater than those that 

might be expected from natural sources 

(Ator and others, in press). (See p. 10.)

Nitrate concentrations in streams 

of the peninsula reflect the influence of agriculture and other human 

sources. Although nitrate may be derived 

from various sources, concentrations 

in headwater streams on the Delmarva 

Peninsula generally increase with 

increasing proportions of agriculture 

in contributing watersheds (fig. 12).

Concentrations typically are highest in 

areas with permeable, well-drained soils, 

aquifer, and streambed sediments. Areas 

with poorly drained soils often contain 

organic-rich sediments and other conditions that promote denitrification, which 

can reduce the concentrations of nitrate 

entering streams. Water in streams from 

some watersheds with predominantly 

poorly drained soils, however, still has 

elevated concentrations of nitrate because 

of interspersion of land-use and soildrainage characteristics in the watershed 

(fig. 12).

Nitrate concentrations were 

similar in streams and ground water 

because ground-water discharge to 

streams is substantial. Ground-water 

discharge is estimated to constitute more 

than one-half of the flow to most streams 

on the Delmarva Peninsula (Bachman 

and others, 1998). As a result, the median 

concentration of nitrate in streams during 

base flow in the spring was similar to that 

in ground water (fig. 13). Some nitrate in 

ground water can be lost through chemical transformations (denitrification) as it 

passes through the riparian zone and the 

streambed into surface water, and aquatic 

plants and algae in the stream may use 

nitrogen and phosphorus compounds 

for growth (biotic uptake). Streambed 

sediments on the Delmarva Peninsula, 

however, are commonly sandy with low 

organic-matter content so that denitrification does not significantly decrease the 

nitrate content in ground-water discharge 

in many areas (Böhlke and Denver, 1995; 

Speiran, 1996).

Figure 11. Concentrations 

of nitrate were unchanged 

between 1988 and 2001 in shallow ground water in agricultural 

areas; however, concentrations 

increased by an average of 

about 2 mg/L in deeper parts 

of the surficial aquifer used 

for domestic water supply. 

This increase is apparent only 

in well-oxygenated areas of 

the aquifer; concentrations in 

poorly oxygenated areas were 

consistently low.

Figure 12. Nitrate 

concentrations generally 

increased with increasing 

percentage of agriculture in the contributing 

watershed in headwater 

streams during spring 

base flow. 

SHALLOW GROUND WATER

IN AGRICULTURAL AREAS

NITRATE, IN MILLIGRAMS PER LITER

30

20

10

 0

Federal

drinking-water 

standard

1988–1990

1999–2001

EXPLANATION 19 Number of samples

Maximun

Median

Minimun

9

19

17

11

 GROUND WATER USED

 FOR DOMESTIC SUPPLY

0

1

2

3

4

5

6

7

8

9

10

11

12

13

0 10 20 30 40 50 60 70 80 90 100

Watersheds with typically permeable,

well-drained soils

Watersheds with typically poorly

drained soils

NITRATE CONCENTRATION, IN MILLIGRAMS PER LITER

PERCENTAGE OF AGRICULTURAL LAND IN WATERSHED
10 Water Quality in the Delmarva Peninsula, Delaware, Maryland, and Virginia, 1999–2001

Nutrient concentrations on the Delmarva Peninsula 

are among the Nation’s highest

Concentrations of nitrogen and 

phosphorus in streams and shallow 

ground water in agricultural areas of 

the Delmarva Peninsula are comparable to some of the highest measured 

in intensively agricultural areas of the 

Midwest and other parts of the Nation. 

Mean flow-weighted concentrations of 

total nitrogen in Chesterville Branch 

(in the Chester River Basin) and in 

the upper Pocomoke River (northeast 

of Salisbury, Md.) for 2000 and 2001 

exceeded 4 mg/L; total concentrations 

of nitrogen in both streams are greater 

than concentrations in at least 80 percent of 479 streams sampled across the 

Nation by the NAWQA Program since 

1993, although many of these streams 

are in nonagricultural areas and are 

much larger.

Most of the nitrogen reaching 

Delmarva Peninsula streams is transported through ground water in the form 

of nitrate. Nitrate concentrations in water 

from the surficial aquifer in agricultural 

areas of the peninsula typically exceed 

5 mg/L and are among the highest (top 

30 percent) measured in ground water 

underlying agricultural areas of the Nation. 

Nitrate travels relatively unaltered through 

sandy, oxygenated streambed sediments 

into streams in many parts of the Delmarva 

Peninsula. Phosphorus concentrations in 

these streams (particularly the Pocomoke 

River) were similarly elevated (top 40 percent) relative to streams in other parts of 

the Nation. Phosphorus concentrations are 

generally lower in ground water than in 

surface water because the major source of 

phosphorus is overland flow from agricultural fields.

The relatively high nutrient concentrations in streams and ground water 

of the peninsula result from intensive 

application of fertilizers (including 

manure) in agricultural areas as well as 

from hydrogeologic and soil conditions. 

Estimated nitrogen and phosphorus 

inputs to the peninsula from commercial fertilizer, manure, and atmospheric 

deposition are comparable to those 

in the Midwest, the Central Valley of 

California, and other areas of intensive 

agriculture. When not used by crops or 

other terrestrial plants, these nutrients 

are effectively transported with little 

transformation or other losses through 

the typically permeable soils to shallow ground water and streams. Excessive nutrients can make water unfit for 

human consumption and can cause 

eutrophication and other ecological 

problems in streams and estuaries.

Median concentration of nitrate, in milligrams per liter

Each circle represents a ground-water study

Highest (greater than 5 mg/L)

Medium (0.4 to 5 mg/L)

Lowest (less than 0.4 mg/L)

Background concentration

Bold outline indicates median values

greater than background concentration

(2 milligrams per liter)

Average annual total nitrogen input,

in pounds per acre, by county, for 1995−98.

Inputs are from fertilizer, manure, and the atmosphere

Greater than 25

6 to 25

Less than 6 

EXPLANATION

Phosphorus input,

in pounds per acre

Mean Annual Concentration,

in milligrams per liter

Mean Annual Concentration,

in milligrams per liter

Nitrogen input,

in pounds per acre

Less than 2

2 to 5

Greater than 5

Less than 0.05

0.05 to 0.3

Greater than 0.3 Less than 6

6 to 25

Greater than 25

Less than 0.6

0.6 to 3

Greater than 3

Total N in streams, 1993-2001

(Agricultural areas)

Total P in streams, 1993-2001

(Agricultural areas)

NITRATE concentrations in shallow ground water

(Agricultural areas)
Major Findings 11

Ammonia and organic nitrogen 

were higher in surface water than in 

ground water (fig. 13). Specifically, the 

median concentration of ammonia plus 

organic nitrogen in ground water was 

about 0.1 mg/L, whereas the median 

concentration in streams was about 

0.5 mg/L. Ammonia and organic nitrogen are produced in streams from the 

decay of leaves and other natural debris, 

resulting in generally higher concentrations of these chemicals in stream water 

than in ground water. 

Phosphorus is generally present at low concentrations in most 

Delmarva Pennsula streams and typically increases during storms. Concentrations of phosphorus in Delmarva 

streams during base flow were typically 

less than 0.1 mg/L, the recommended 

level to prevent excessive plant growth in 

freshwater streams (U.S. Environmental 

Protection Agency, 1986). Concentrations were as high as 3 or 4 mg/L, however, during base flow in some streams 

in poorly drained areas with abundant 

organic matter and low dissolved-oxygen 

concentrations.

Concentrations of phosphorus 

generally increase during storm runoff 

and may exceed 1 mg/L in streams 

throughout the peninsula. For example, 

concentrations of sediment and total 

phosphorous increased with increasing streamflow at Chesterville Branch 

from storm runoff; concentrations of 

nitrate and other compounds that are 

carried primarily through ground water 

decreased because of dilution by storm 

runoff (fig. 14) (Ator and others, in 

press).

Figure 13. Although ground water from 

the surficial aquifer is the primary source 

of water to small streams during base flow, 

concentrations of nitrogen species often 

differ in surficial ground water and small 

streams during such conditions. Nitrate 

concentrations were typically higher in 

ground water (represented by water from 

the part of the surficial aquifer used for 

domestic drinking-water supplies), whereas 

concentrations of ammonia plus organic 

nitrogen, which are less soluble forms of 

nitrogen, were typically higher in streams.

Figure 14. Changes in nutrient concentrations with streamflow reflect different modes 

of transport for nitrate and phosphorus to Chesterville Branch. The marked increase in 

suspended-sediment concentrations indicates that overland runoff becomes significant 

in the watershed when flow reaches about 10 cubic feet per second. Phosphorus is 

relatively insoluble, and the highest concentrations are associated with sediment during 

high flow. Nitrate, however, is fairly soluble. Decreasing nitrate concentrations with increasing flow indicate that nitrate moves to Chesterville Branch primarily through ground water.

Nutrient concentrations in 

agricultural streams on the Delmarva 

Peninsula vary seasonally. Nitrate concentrations in surface water typically are 

greatest during the winter and least in the 

summer. For example, from 1999 through 

2001, concentrations of nitrate in the 

Pocomoke River reached a maximum of 

5 mg/L as N in January but were lowest 

in June and July (fig. 15). This seasonality can be explained by hydrologic conditions and biologic uptake. Specifically, 

during winter and spring, the water table 

is highest, and shallow ground water 

containing nitrate contributes a larger 

proportion of base flow. In addition, 

biotic uptake of nitrate is relatively 

low because of cool temperatures. 

As a result, concentrations of nitrate 

in stream water are high (fig. 15). 

Lower nitrate concentrations in the 

Pocomoke River during the dry season 

(summer and fall) most likely reflect 

a greater proportion of flow from the 

underlying confined aquifer, which 

contains no nitrate. In addition, biotic 

uptake of nitrate is at its peak in the 

warmer, drier summer months.

Base-flow nitrate concentrations 

are expected to increase in Delmarva 

streams and downstream estuaries 

in the coming years. As water with 

elevated concentrations of nitrate currently in the surficial aquifer upgradient from surface-water discharge areas 

is transported to streams (fig. 10), an 

increase in nitrate concentrations in 

stream base flow—and therefore in the 

total load of nitrate reaching downstream estuaries—is likely. Increased 

biotic uptake may partially mitigate 

nitrate concentrations in some streams, 

although the increased plant growth 

may contribute to eutrophication or 

other ecological problems.

0

10

20

30 29

23

Maximum

Number of samples

Median

Minimum

29

Ground water

Streams

EXPLANATION

0.01

0.1

1

10

29

23

NITRATE CONCENTRATION,

IN MILLIGRAMS PER LITER

AMMONIA PLUS ORGANIC 

NITROGEN CONCENTRATION,

IN MILLIGRAMS PER LITER

29

23

23

29

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12 Water Quality in the Delmarva Peninsula, Delaware, Maryland, and Virginia, 1999–2001

Pesticides in Ground Water 

and Streams

Approximately 3.8 million 

pounds of pesticides was applied in 

1997 on the Delmarva Peninsula solely 

for agricultural purposes, mostly on 

corn (1.6 million pounds) and soybeans (997,000 pounds) (Gail Thelin, 

U.S. Geological Survey, written commun., 2002). Metolachlor, atrazine, 

glyphosate, alachlor, and simazine were 

the most widely used herbicide compounds in 1997, the last year for which 

summary data are currently available 

(fig. 16, table 1). Only 28 percent 

of the total agricultural pesticide 

applications on the peninsula are 

insecticides, fungicides, or other nonherbicides (fig. 16). Most of this use 

is of methyl bromide, a soil fumigant 

applied to tomatoes and strawberries 

in the Virginia part of the peninsula 

(Gail Thelin, written commun., 2002). 

Although it is applied in only a limited 

area, methyl bromide is applied at 

much higher rates than other commonly used pesticide compounds.

Pesticides also are applied annually 

for residential, transportation, industrial, 

public health, or other nonagricultural 

purposes. About one-fourth of pesticide applications nationally are for 

nonagricultural purposes (Aspelin and 

Grube, 1999).

Herbicides are frequently 

detected in the surficial 

aquifer 

Pesticides are present at low 

concentrations in water from the 

surficial aquifer throughout the 

Delmarva Peninsula (fig. 17). Concentrations of individual compounds detected 

by NAWQA were typically less than 

1 μg/L and were always below current 

Federal drinking-water standards and 

health advisories (U.S. Environmental 

Protection Agency, 2002a); however, 

standards or advisories are available for 

only 42 of the 119 pesticides analyzed 

for in this study.

Pesticides are widespread in shallow ground water underlying agricultural areas. Pesticides were detected 

in water samples from 25 of 29 shallow 

wells (median depth 22 ft) in agricultural 

areas. Twenty different pesticide compounds, mostly herbicides used on crops, 

were detected. Metolachlor and atrazine, 

which have been widely used since the 

1970s, were the most commonly detected 

herbicides. Flumetsulam, imazaquin, 

imazethapyr, and glyphosate, which have 

increased in use in the last 10 years, also 

were detected, but less frequently. Less 

than 20 percent of samples contained 

detectable prometon or bromocil, which 

are applied to brush along roadways. 

Insecticides and fungicides used on 

crops also were detected less frequently.

Pesticides also are common 

in ground water used for domestic 

supply (median well depth 45 ft). Types 

of compounds were similar to those in 

shallow ground water in agricultural 

areas (fig. 17). Metolachlor and atrazine were the most widely detected 

compounds. Twelve other herbicides 

also were detected, including the more 

recently used pesticides flumetsulam, 

Figure 16. Applications 

of herbicides typically 

far exceed those of 

other types of pesticides 

in agricultural areas on 

the Delmarva Peninsula, 

as shown by total usage 

of the top 15 pesticides. 

(Data from Gail Thelin, 

U.S. Geological Survey, 

written commun., 2002.)

Figure 15. Nitrate concentrations were typically lowest in the Pocomoke River during 

June and July and highest during January and February during the sampling period from 

1999 through 2001. Higher concentrations during the winter reflect greater discharge 

of water from the surficial aquifer, whereas lower concentrations during the summer 

reflect the increased proportion of discharge from the underlying confined aquifer, plus 

biotic uptake.

NITRATE CONCENTRATION,

IN MILLIGRAMS PER LITER

0

1

2

3

4

5

 JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEPT

OCT

NOV

DEC

COMPOUND

2,4-D

Cyanazine

Pendimethalin

Acetochlor

Paraquat

Simazine

Alachlor

Glyphosate

Atrazine

Metolachlor

Mancozeb

Chlorothalonil

Terbufos

Chloropicrin

Methyl bromide

TOTAL USE 1997 (IN 100,000 POUNDS ACTIVE INGREDIENT)

0 1 2 3 4 5 6 7 8 9 10 11 12 

Herbicides

Fungicides

Insecticides

Other pesticides
Major Findings 13

imazaquin, glyphosate, and imazethapyr. Fungicides used on crops and 

insecticides generally used for controlling insects around buildings were less 

frequently detected.

Pesticides also are present in 

the deeper part of the surficial aquifer 

used for public supply. Samples from 

the majority (27 of 30) of sampled 

public-supply wells completed in the 

surficial aquifer in Delaware (median 

depth 80 ft) contained detectable pesticides. (These samples were collected 

before any treatment and therefore 

may not represent the quality of tap 

water.) Thirteen different pesticides 

were detected. The suite of pesticides 

detected in the public-supply wells 

reflects a combination of agricultural 

and urban land use in aquifer recharge 

areas, as well as historical application 

rates. Although the most commonly 

used agricultural chemicals, metolachlor 

and atrazine, were the most frequently 

detected in water from public-supply 

wells, chemicals more frequently used in 

urban areas also were detected (fig. 17). 

These include chemicals used for weed 

control along roads, such as prometon 

and tebuthiuron, and those used historically for insect control around buildings, such as dieldrin and lindane. The 

other crop pesticides detected include 

the herbicides alachlor and simazine 

and the insecticide carbofuran. Samples 

from the public-supply wells were not 

analyzed for some of the more recently 

used pesticides, including flumetsulam, 

imazaquin, imazethapyr, and glyphosate.

Mixtures of pesticides and pesticide degradates typically are present in the surficial aquifer. Shallow 

ground water in agricultural areas and 

in parts of the aquifer used for domestic supply commonly contain mixtures 

of metolachlor, alachlor, atrazine, and 

their degradates, often in combination with other less frequently detected 

herbicide or insecticide compounds. 

More than 85 percent of water samples 

(67 of 78) with detectable pesticides or 

degradates contained three or more different compounds (fig. 18). Most of the 

chemical mixtures reflected agricultural 

uses. Water from public-supply wells 

contained similar mixtures of the most 

widely used crop herbicides as well as 

some herbicides and insecticides more 

commonly used in urban areas. Samples 

from many of the public-supply wells 

contained a greater total number of pesticides or pesticide degradates than the 

shallow agricultural and domestic wells 

sampled; one sample contained 15 different compounds. The potential additive or synergistic effects of low levels 

of multiple compounds on human health 

or aquatic life are unknown. Current 

experience and research on the environmental and toxicological effects of 

pesticides is usually based on exposure 

to individual compounds.

Degradates of some herbicides typically are present at higher 

concentrations than their parent 

compounds in the surficial aquifer.

For example, median concentrations of 

the degradates alachlor ESA (ethanesulfonic acid) and metolachlor ESA 

Figure 17. Although metolachlor and 

atrazine and their degradates are the most 

frequently detected compounds in ground 

water, pesticides associated with urban 

land use are more frequently detected in 

water from deeper parts of the aquifer in 

public-supply wells than in shallower parts 

of the surficial aquifer. Pesticides typically 

are detected more frequently in surface 

water than in ground water.

DETECTION FREQUENCY, IN PERCENT 

COMPOUND

0 10 20 30 40 50 60 70 90 80 100

Headwater streams during spring base flow

Ground water used for domestic supply Ground water used for public supply

Shallow ground water in agricultural areas 

,

Table 1. Commonly used herbicides on the Delmarva Peninsula.

Herbicide Thousands of 

 pounds used, 1997 Crops

Metolachlor 1,145 Corn, soybeans

Glyphosate 235 Soybeans

Atrazine 500 Corn

Simazine 94 Corn

From U.S. Department of Agriculture, 1999

Alachlor 134 Corn, soybeans
14 Water Quality in the Delmarva Peninsula, Delaware, Maryland, and Virginia, 1999–2001

generally are greater than 0.1 μg/L, 

whereas median concentrations of 

their parent compounds, alachlor and 

metolachlor, are about 10 times less 

(about 0.01 μg/L) (fig. 19). These two 

compounds are typical of most applied 

pesticides, only a low percentage of 

which reaches ground water because 

of adsorption onto organic matter and 

microbial degradation of the parent 

compound in the soil zone (Helling and 

Gish, 1986; Capel and others, 2001). In 

contrast, concentrations of atrazine and 

its degradation product, deethylatrazine, 

commonly occur at similar concentrations in ground water. This finding can 

be explained, in part, by the chemical 

properties of atrazine. Specifically, a 

combination of relatively high mobility and chemical stability in the soil 

zone allows atrazine to move to, and 

persist in, the ground-water flow system 

longer than metolachlor and alachlor, 

which have lower persistence in the 

soil zone and tend to break down more 

completely to degradates before reaching the ground-water system. Little is 

known about the occurrence of many 

other pesticide breakdown products, and 

even less is known about their effects on 

human health and aquatic life. There are 

no Federal standards or guidelines for 

these pesticide degradates.

Pesticides vary in the surficial 

aquifer because of land use, changes 

in pesticide usage over time, and 

physical and chemical properties that 

control their movement to ground 

water. The occurrence and concentrations of agricultural pesticides in the 

surficial aquifer are controlled by several 

factors. First, pesticide occurrence in 

shallow ground water varies with land 

use; concentrations detected in a well 

generally increase as the percentage of 

agricultural land use within 500 meters 

CONCENTRATION, IN MICROGRAMS PER LITER

ALACHLOR

METOLACHLOR ESA

DEETHYLATRAZIN

METOLACHLOR

E

ATRAZINE

ALACHLOR ESA

Shallow ground water in agricultural areas

Ground water used for domestic supply

Ground water used for public supply

EXPLANATION 29 Number of samples Maximum

Median

Minimun

 100

 10

 1

 0.1

 0.01

 0.001

0.0001

Figure 18. Multiple 

pesticide compounds 

were typically detected in 

individual water samples 

from the surficial aquifer. 

Three or more compounds 

were detected in more 

than 85 percent of the 

wells sampled. 

Figure 19. Degradates of 

metolachlor and alachlor 

frequently were present 

in water from the surficial 

aquifer at higher concentrations than the parent compounds; concentrations of 

atrazine and its degradate, 

deethylatrazine, are similar.

What is a pesticide 

degradate?

Pesticides released into the 

environment break down into intermediate compounds, called degradates, 

and over time into simple molecules 

like carbon dioxide and water. The 

breakdown of pesticides occurs both 

through abiotic and biotic processes. 

Abiotic processes involve photochemical or chemical transformations that 

require sunlight or naturally occurring 

chemicals in soil and water. Biotic 

processes involve transformations by 

animals, plants, and microbes. Breakdown occurs primarily in the soil, 

but it also can occur in plants and in 

ground water and streams. Modification of a side carbon chain (such as in 

atrazine degradation) or the replacement of the halogen (chlorine) with a 

sulfonic acid or carboxylic acid (such 

as in acetochlor, alachlor, or metolachlor degradation) is commonly the 

first step in the breakdown of these 

herbicides. This first step represents 

only a small molecular change; 

however, this change may affect the 

toxicity, mobility, and persistence of 

the resulting compound compared to 

its parent compound.

Degradates may be short-lived 

in the environment or persist for 

years. Some may be retained in the 

soil, whereas others may move easily with water through the ground 

and to streams. A wide variety of 

degradates can form from a single 

parent compound. For example, the 

most widely used pesticide on the 

Delmarva Peninsula, metolachlor, 

has two commonly detected degradates, metolachlor ethanesulfonic 

acid (ESA) and metolachlor oxanilic 

acid (OA). Other common pesticides, 

such as atrazine, have as many as 

four major degradates, including the 

most common one, deethylatrazine; 

and there are many more for which 

analytical techniques have not been 

developed.

NUMBER OF PESTICIDES AND DEGRADATES DETECTED

NUMBER OF GROUND-WATER SAMPLES
Major Findings 15

of the well increases (fig. 20). Second, 

types of pesticides found in the surficial 

aquifer strongly reflect chemical use. 

For example, commonly used metolachlor, alachlor, atrazine, and their 

degradates were the most frequently 

detected compounds in this study, 

although newer crop herbicides such as 

flumetsulam, imazaquin, and imazethapyr also were present in shallower parts 

of the aquifer where recharge has been 

recent (typically since the early 1990s) 

(fig. 17). Acetochlor was not detected in 

ground water on the peninsula, possibly because it has been used only since 

1994. Historically used pesticides, such 

as carbofuran, were more frequently 

found in deeper and older ground water 

(recharged around 1985), such as that 

collected from public-supply wells, 

than in shallower, near-surface ground 

water recharged since 1994. Third, 

the frequency of detection and concentrations of selected pesticides are 

controlled by chemical and physical 

properties. For example, concentrations of herbicides and their degradates 

are higher in oxygen-rich environments, where soils are well drained 

and permeable with low organic-matter 

content, than in poorly drained areas 

with typically higher concentrations of 

organic matter, lower concentrations of 

dissolved oxygen, and a greater potential for microbial degradation in the 

soil zone. Glyphosate, the third most 

commonly used herbicide in 1997, was 

seldom detected, probably because it is 

strongly adsorbed onto soil particles.

Herbicides and their degradates 

have persisted in the surficial aquifer for more than 30 years (fig. 21). 

Data collected along a ground-water 

flow path at an agricultural study area 

in Fairmount, Del., show atrazine, 

metolachlor, and their degradates in 

water that recharged the aquifer as early 

as the mid-1960s. Degradates of both 

herbicides persist in the aquifer at higher 

concentrations than do their parent 

compounds. As discussed previously, 

most metolachlor degradation occurs 

in the soil zone. Although atrazine also 

breaks down in the soil zone, a decrease 

in atrazine and a corresponding increase 

in deethylatrazine over time implies 

continued degradation of atrazine as 

water moves through the ground-water 

system.

Herbicide concentrations in 

water from the surficial aquifer used 

for domestic supply have changed little 

over the past decade, and remain low. 

Although metolachlor and atrazine were 

detected more frequently in ground water 

in 2001 than in 1988 or 1990, this likely 

is due more to the current availability of 

sensitive laboratory techniques for analyzing water samples rather than to any 

apparent increase in concentrations within 

the aquifer. Twenty-three of the 29 wells 

sampled in 2001 to represent the groundwater resource used for domestic supply 

also were sampled in either 1988 or 1990 

(see Study Unit Design, p. 22–23). Only 

one of these 23 samples contained detectable (greater than 0.007 μg/L) atrazine 

in 1988, and none contained detectable 

(greater than 0.018 μg/L) metolachlor 

(Koterba and others, 1993). Concentrations of atrazine exceeded 0.007 μg/L 

in samples from 3 of these 23 wells in 

2001, and atrazine was detected at lower 

concentrations in samples from one other 

well. Similarly, samples from four wells 

contained detectable metolachlor in 2001, 

although the concentration in only one 

exceeded 0.018 μg/L.

Temporal trends in herbicide 

concentrations are similarly unclear 

or insignificant in shallow ground 

water beneath agricultural areas. 

Fifteen of the 29 wells sampled in 2001 

to represent this resource also were sampled in either 1988 or 1990 (see Study 

Unit Design, p. 22–23). Concentrations 

of metolachlor exceeded 0.018 μg/L 

in only 2 of the 15 samples in 1988 

or 1990, and only 3 samples contained 

detectable atrazine (Koterba and others, 

1993). In 2001, samples from nearly 

one-half (7) of these 15 wells contained 

detectable metolachlor, although (as in 

1988 and 1990) concentrations exceeded 

0.018 μg/L in only 2 samples. Six of 

the 15 samples contained detectable 

atrazine, although concentrations in 

only 4 samples exceeded 0.007 μg/L.

TOTAL MEASURED PESTICIDE CONCENTRATION,

IN MICROGRAMS PER LITER

AGRICULTURE, IN PERCENTAGE OF AREA

WITHIN 500 METERS OF SAMPLED WELL

Greater than 1 milligram per liter

Less than 1 milligram per liter

Dissolved oxygen

Figure 20. The total measured 

concentration of pesticides 

generally increased in shallow 

ground water beneath agricultural areas with increased 

intensity of agricultural activity 

in well-oxygenated parts of 

the surficial aquifer (dissolvedoxygen concentration greater 

than 1 mg/L).

 0

0.25

0.50

0.75

1.00

(3.59 μg/L)

0

10

20

30

40

0 10 20 30 40

0 10 20 30 40

GROUND-WATER AGE (YEAR)

CONCENTRATION, IN MICROGRAMS PER LITER

Metolachlor

Metolachlor ESA

Metolachlor OA

Concentration below

reporting limit

Atrazine

Deethylatrazine

Concentration below

reporting limit

Figure 21. Concentrations of atrazine, 

metolachlor, and their degradates have 

persisted along a ground-water flow path 

at an agricultural study site in Fairmount, 

Delaware, for more than 30 years.
16 Water Quality in the Delmarva Peninsula, Delaware, Maryland, and Virginia, 1999–2001

Pesticides are commonly 

detected in streams

Pesticides and their degradates 

are more frequently detected and 

generally at higher concentrations 

in streams than in water from the 

surficial aquifer (fig. 22, fig. 17). Of the 

55 pesticides for which both stream and 

ground-water samples were analyzed, 33 

were detected in streams and only 20 in 

ground water. Metolachlor and atrazine 

and their degradates were the most 

frequently detected in streams during 

spring base flow (fig. 17). Acetochlor 

and its degradates, which were seldom 

detected in ground water, also were 

detected more commonly in streams. 

Prometon and simazine, which are used 

for weed control along roads and other 

rights-of-way, also were more frequently 

detected in surface water.

Concentrations of pesticides 

and degradates were typically higher 

in streams during base flow than in 

ground water—sometimes by as much 

as 10 times, as indicated by the commonly used herbicides metolachlor and 

atrazine (fig. 22). Greater concentrations 

of pesticide compounds in streams than 

in ground water may be due to transport 

along short, shallow flow paths that 

do not allow much time for chemical 

changes. Also, pesticides are contributed 

as they are desorbed from sediments 

that were transported to streams during 

storms.

Concentrations of individual pesticides in streams rarely exceeded levels 

that are known to be harmful to aquatic 

plants and animals, although such levels 

are known for only 40 percent of the 

compounds analyzed for in this study. 

Also, largely unknown are the ecological effects of pesticide degradates or of 

mixtures of compounds.

Most pesticides detected in surface water on the Delmarva Peninsula 

are herbicides applied primarily for 

corn and soybean production, such 

as atrazine, metolachlor, and alachlor. The occurrence of other pesticide 

compounds in certain streams, however, 

reflects varying influences of other types 

of land uses. For example, the high 

frequency of detection of prometon, a 

herbicide used primarily along roads and 

other rights-of-way, during spring base 

flow indicates that non-crop sources 

of pesticides also may be important on 

the peninsula. In addition, the frequent 

detection of the herbicide pronamide, 

which is commonly used on ornamental shrubs, as well as the insecticides 

chlorpyrifos, carbofuran, diazinon, and 

carbaryl at Chesterville Branch (see 

Appendix, p. 27–28), suggests that 

applications to nursery plants may be 

affecting stream chemistry in some areas 

(Ator and others, in press).

Mixtures of pesticides are common in headwater streams. At least five 

pesticides or degradates were detected in 

each headwater stream sampled during spring base flow (fig. 23). The most 

common combination of pesticides 

detected was metolachlor, alachlor, 

atrazine, and their degradates, along with 

prometon and simazine. Many other herbicide and insecticide compounds were 

detected less frequently. The number of 

pesticide compounds in streams generally 

increases with increasing percentage of 

agriculture in contributing watersheds. 

In some watersheds, however, urban land 

use also contributes many different pesticides, and in one such watershed 17 different pesticide compounds were detected 

(see data point at upper left in fig. 23).

CONCENTRATION, IN MILLIGRAMS PER LITER

METOLACHLOR ESA

METOLACHLOR

DEETHYLATRAZINE

ATRAZINE

Headwater streams during spring base flow

Ground water used for domestic supply

METOLACHLOR OA

EXPLANATION Number of samples

 Maximum

 Median

 Minimum

29

100

10

1

0.1

0.001

0.01

29

29

29

29

29

23

23

23

23

23

Figure 22. Concentrations 

of the most commonly used 

herbicides, atrazine and metolachlor, and their degradates 

were higher in stream water 

than in ground water (represented by wells from the part 

of the surficial aquifer used 

for domestic supply) on the 

Delmarva Peninsula.

Figure 23. The number of pesticide 

compounds detected generally increases 

with increasing proportions of agriculture 

in contributing watersheds in headwater 

streams during spring base flow.

0 10 20 30 40 50 60 70 80 90 100

NUMBER OF COMPOUNDS DETECTED

PERCENTAGE OF AGRICULTURAL LAND IN WATERSHED

10

15

20

5

0

Concentrations of herbicides are 

generally highest during the spring 

and early summer, soon after periods 

of application. Herbicides are detectable throughout the year in streams in 

agricultural areas of the peninsula, in 

large part because of relatively continuous contributions from ground water 

(Shedlock and others, 1999). However, 

streams also receive pesticides in runoff 

from the land surface. Concentrations 

vary seasonally because pesticidecarrying runoff is most frequent and 

greatest in the spring when herbicides 

are applied, resulting in seasonally 

elevated pesticide concentrations. For 

example, in the northern Delmarva 

Peninsula at Chesterville Branch during 

the 1999–2001 sampling period, concentrations of metolachlor and atrazine, 

which are applied just before planting, 

peaked from March through May during 

the beginning of the growing season 

(fig. 24) (Ator and others, in press). 
Major Findings 17

(particularly degradates) also move 

through the surficial aquifer in these 

areas, however, particularly where soils 

and shallow sediments are sandy.

The discharge of pesticides (which 

is the amount being transported in a 

stream per unit time) increases as streamflow increases. At Chesterville Branch, 

for example, when streamflow is less than 

10 cubic feet per second, ground water 

contributes most of the water and chemicals in the stream. When streamflow is 

greater than 10 cubic feet per second, 

however, discharge of metolachlor, in particular, increases as runoff from agricultural fields increases (fig. 25) (Ator and 

others, in press).

The ecosystem of the Chesapeake 

Bay, the largest and one of the most 

ecologically diverse estuaries in the 

Nation, has been adversely affected by 

a combination of poor water quality 

and overharvesting of living resources, 

such as blue crabs and finfish (Phillips, 

2001). About 75 percent of the land 

area of the Delmarva Peninsula lies 

within the Chesapeake Bay watershed, 

and the NAWQA study on the peninsula is providing critical information 

for understanding water quality in the 

bay and for implementing strategies to 

restore the ecosystem.

collected in rivers and the Chesapeake 

Bay show that nutrient and pesticide compounds, including pesticide 

degradates, that are present in rivers 

are present at similar or slightly lower 

concentrations in the bay. Resource 

managers use this information to help 

target and implement actions to reduce 

the amount of contaminants entering 

the bay. 

Delmarva Peninsula NAWQA study contributes to understanding of water quality

in the Chesapeake Bay

The Chesapeake Bay is affected by 

eutrophication (high concentrations of 

nutrients, leading to blooms of algae and 

other plants) and hypoxia (areas with low 

dissolved-oxygen concentrations), resulting in fishkills and loss of other living 

resources. Decreased water clarity resulting from algal blooms and sediment 

input has caused a decline in submerged 

aquatic vegetation, which is critical habitat for shellfish and finfish and provides 

food for waterfowl. Fish health also is 

threatened by toxic contaminants entering the system from various land uses.

On the peninsula, nutrients such 

as nitrogen and phosphorus and pesticides from agricultural and other human 

sources affect downstream waters. These 

contaminants are transported to the bay 

through overland runoff, from point 

sources in urban areas, and in groundwater discharge. In fact, ground-water 

discharge may contribute more than 

one-half of the load of nitrate delivered to the bay (Bachman and others, 

1998). Hydrologists in the Delmarva 

Peninsula Study Unit coordinated with 

other USGS scientists working in the 

bay watershed to better quantify the 

time that elapses between ground-water 

recharge and discharge (ground-water 

residence times). Water-quality data 

Local differences in geology, 

topography, and soils affect the 

movement of pesticides to streams.

Although pesticides are transported to 

streams from fields by both overland 

runoff and the ground-water system 

throughout the peninsula, each of these 

processes may be more or less important 

in different areas. In flat areas, such as 

the south-central part of the peninsula, 

overland runoff is rare, and pesticides are 

probably transported to streams primarily through the ground-water system. 

Topographic relief is generally greater 

in the northern part of the peninsula, and 

overland runoff during storms is probably more frequent. Pesticide compounds 

0.01

0.10

1.00

10.00

CONCENTRATION, 

IN MICROGRAMS PER LITER

Jan Mar May July Sept Nov

Atrazine

Metolachlor

Figure 24. Although atrazine and metolachlor were detectable throughout the year 

in Chesterville Branch (a stream draining an 

agricultural watershed near Kennedyville, 

Maryland) during the 1999–2001 sampling 

period, concentrations were typically highest during application periods in the spring.
18 Water Quality in the Delmarva Peninsula, Delaware, Maryland, and Virginia, 1999–2001

Herbicide concentrations in streams and ground water in 

agricultural areas on the Delmarva Peninsula are similar to those in 

other intensively farmed areas where corn and soybeans are grown

Herbicide concentrations in streams and shallow ground water in agricultural areas of the Delmarva 

Peninsula are similar to concentrations in the Upper 

Midwest and other heavily farmed areas of the Nation. Soils 

and shallow sediments in agricultural areas of the peninsula 

are typically sandy, permeable, and well weathered, and 

applied herbicides are apparently transported effectively to 

ground water and streams. Although herbicide application 

rates are higher in areas such as the Midwest, many of those 

areas contain relatively impermeable organic-rich soils that 

may more effectively retain applied herbicides. Established 

guidelines for the protection of aquatic life were occasionally exceeded for at least one compound in Chesterville 

Branch.

The occurrence of specific herbicides on the Delmarva 

Peninsula reflects local application rates and patterns as 

well as water solubility and local hydrologic, geologic, 

and soil conditions. Detection frequencies for atrazine in 

ground water of the peninsula were similar to those in other 

areas of the Nation, although atrazine was detected less 

frequently in Chesterville Branch and the upper Pocomoke 

River than in similar streams nationally. Metolachlor, on 

the other hand, was detected more frequently in surficial 

ground water in agricultural areas of the peninsula than in 

similar areas of the Nation and was measured at concentrations exceeding 0.1 μg/L in 39 of 40 samples collected from 

Chesterville Branch. Concentrations of metolachlor in the 

upper Pocomoke River were similar to those in other agricultural streams around the Nation. Metolachlor is the most 

heavily used herbicide on the peninsula. Moreover, metolachlor is used on both corn and soybeans, whereas atrazine is 

used only on corn.

FREQUENCY OF COMPOUND DETECTION

AT CONCENTRATIONS GREATER THAN

0.1 μg/L (IN PERCENT)

0 20 40 60 80 100

AGRICULTURAL STREAMS NATIONALLY

CHESTERVILLE BRANCH NEAR 

CRUMPTON, MD.

POCOMOKE RIVER NEAR WILLARDS, MD.

HEADWATER STREAMS ON THE DELMARVA

PENINSULA DURING SPRING BASE FLOW

GROUND WATER IN AGRICULTURAL

 AREAS NATIONALLY

GROUND WATER IN AGRICULTURAL

 AREAS, DELMARVA PENINSULA

GROUND WATER IN MAJOR

AQUIFERS NATIONALLY

GROUND WATER USED FOR DOMESTIC

SUPPLY, DELMARVA PENINSULA

GROUND WATER USED FOR PUBLIC

SUPPLY, DELMARVA PENINSULA

Atrazine

Metolachlor

Total samples

,

,

,

,

,

,

Sum of herbicide concentrations in the GCMS 

method — 75th percentile, in micrograms per liter

Agricultural streams

Aquatic-life guidelines

Highest 25 percent (greater than 1.070)

Middle 50 percent (0.093 to 1.070)

Lowest 25 percent (less than 0.093)

Herbicide use, in pounds 

per acre of agricultural land*

Highest (greater than 0.406)

Medium (0.090 to 0.406)

Lowest (less than 0.090)

No reported use

*Data available for conterminous US only

24

23

36

30

41

21

20

22

20

Bold outline indicates exceedance by one or more

herbicides in the GCMS method. Number is percentage

of samples that exceeded a guideline within a oneyear period. Percentages less than 20 are not labeled.

Frequency of wells with one or more herbicides in the

GCMS method — Each circle represents a ground-water study 

Agricultural areas

Herbicides in Shallow Ground Water

Drinking-water standards or guidelines

Bold outline indicates exceedance by one or more

herbicides in the GCMS method. Number is

percentage of wells that exceeded a standard or

guideline.

Highest 25 percent (greater than 59.2)

Middle 50 percent (16.2 to 59.2)

Lowest 25 percent (less than 16.2)

Herbicide use, in pounds 

per acre of agricultural land*

Highest (greater than 0.406)

Medium (0.090 to 0.406)

Lowest (less than 0.090)

No reported use

3

3

*Data available for conterminous US only

EXPLANATION EXPLANATION

Herbicides

GCMS, Gas chromatography/Mass spectrometry
Major Findings 19

Other Ground-Water 

Quality Issues

Volatile organic compounds 

are detected in ground water

Volatile organic compounds 

(VOCs), such as components of fuels, 

lubricants, and solvents, are present 

in water from the surficial aquifer on 

the Delmarva Peninsula (fig. 26). At 

least one VOC was detected in each 

sample collected from 30 publicsupply wells screened in the surficial 

aquifer in Delaware in 2000 (Ferrari, 

2001). Water from most wells contained 

multiple VOCs, including four wells in 

which more than 10 compounds were 

detected. Of the 85 different VOCs 

for which samples were analyzed, 34 

were detected at least once. Measured 

concentrations were typically less 

than 1 μg/L, which is below existing Federal standards and guidelines 

for the protection of drinking water 

(U.S. Environmental Protection Agency, 

2002a,b). No standards and guidelines 

have been established, however, for 

22 of the 34 compounds detected. The 

high frequency of VOC detection in the 

public-supply wells reflects the ready 

transport of organic chemicals through 

the sandy, permeable sediments of the 

surficial aquifer.

Chloroform was the most 

frequently detected VOC in publicsupply wells (28 of 30) (fig. 26). Chloroform is used in some industrial and 

manufacturing processes, and it is commonly produced during the chlorination 

of drinking water or as a byproduct of 

the chlorination of wells to kill bacteria 

that may be introduced during drilling. 

The next most frequently detected VOC, 

tetrachloroethene, was detected in 20 of 

the public-supply wells. This chemical is 

used in some industrial and manufacturing processes as well as in dry cleaning. 

Methyl tert-butyl ether (MTBE), the 

third most frequently detected VOC, 

is widely used as an additive in reformulated gasoline to increase the octane 

level and reduce carbon monoxide emissions from vehicles; it was detected in 

17 samples from public-supply wells on 

the Delmarva Peninsula.

Only nine different VOCs were 

detected in water from 16 of 28 wells 

completed in the part of the surficial aquifer used for domestic water 

supply (including domestic and 

monitoring wells). The lower number 

of VOCs in these wells compared to 

those detected in public-supply wells is 

expected because of fewer sources of 

VOCs in rural areas compared to urban 

areas (fig. 26). Multiple VOCs were 

detected in eight of the wells, with a 

maximum of five different compounds 

detected in water from one well. Concentrations generally did not exceed 

0.1 μg/L and were not above any existing Federal drinking-water standards 

and guidelines (U.S. Environmental 

Protection Agency, 2002b). As in 

public-supply wells, chloroform was the 

most frequently detected compound (14 

of 28 wells). The detection frequency of 

chloroform (50 percent) in the part of the 

surficial aquifer used for domestic supply is higher than the national detection 

frequency (30 percent) for wells in other 

major aquifers sampled. The next most 

frequently detected VOCs in the part 

of the surficial aquifer used for domestic supply were 1,1-dichloroethane (a 

solvent) and carbon disulfide (which has 

numerous uses).

VOCs can reach ground water from 

various sources, including chemical manufacturing, industrial processes (solvents, 

degreasers, dry cleaning), gasoline or 

other petroleum-based fuels, byproducts 

of the chlorination of drinking water, or 

agricultural applications (used as either 

pesticides or solvents for pesticides).

Radon occurs in water 

throughout the surficial 

aquifer

Radon was detected in all 

63 water samples from the surficial 

aquifer for which it was analyzed. 

Radon activity exceeded 300 pCi/L 

(picocuries per liter), the proposed 

Federal maximum contaminant level 

Figure 25. Discharge of both 

metolachlor and metolachlor 

ESA increases with increasing streamflow in Chesterville 

Branch. These compounds are 

probably carried to the stream 

primarily through ground water 

when streamflow is less than 

10 cubic feet per second. Additional quantities of these compounds are carried in surface 

runoff during higher flows.

Figure 26. Volatile organic 

compounds (VOCs) were 

detected in all 30 samples 

from public-supply wells 

and in 16 of 28 wells completed in the part of the 

surficial aquifer used for 

domestic supply.

1

10

100

1,000

10,000

STREAMFLOW, IN CUBIC FEET PER SECOND

1 10 100

PESTICIDE DISCHARGE, 

 IN MICROGRAMS PER SECOND

Metolachlor

Metolachlor ESA

DETECTIONS, IN PERCENT 

COMPOUND

0 10 20 30 40 50 60 70 80 90 100

1,2-Dichlorobenzene

Benzene

Carbon disulfide

cis-1,2-Dichloroethene

1,1-Dichloroethane

1,1-Dichloroethene

Methyl tert-pentyl ether

Toluene

1,1,1-Trichloroethane

Chloromethane

1,1,2-Trichlorotrifluoroethane

Methyl tert-butyl ether (MTBE)

Tetrachloroethene

Chloroform

Trichloroethene

Ground water used

for domestic supply

Public-supply wells
20 Water Quality in the Delmarva Peninsula, Delaware, Maryland, and Virginia, 1999–2001

(U.S. Environmental Protection Agency, 

2002a), in samples from 22 percent (6 of 

27) of wells from the part of the surficial 

aquifer used for domestic supply but 

only in 1 of the 10 public-supply wells 

sampled for radon. Measured radon activities were as high as 1,520 pCi/L. Radon 

concentrations in water from the surficial 

aquifer on the Delmarva Peninsula are 

typical of those in other Coastal Plain 

aquifers, such as in Delaware (Bachman 

and Ferrari, 1995), Maryland (Bolton 

and Hayes, 1999; Drummond, 2001) 

and New Jersey (Kozinski and others, 

1995) and generally are lower than those 

in other unconsolidated or consolidated aquifers around the Nation (see 

Appendix, p. 30).

Radon is a naturally occurring, 

colorless, odorless gas derived from the 

radioactive decay of uranium in rocks and 

soil. It has been identified as a carcinogen 

in humans, primarily as a cause of lung 

cancer. The source material for the sediments on the Delmarva Peninsula that 

contain uranium are rocks in adjacent 

areas of Pennsylvania, Virginia, and 

West Virginia (Lindsey and Ator, 1996).

Dissolved iron is present in 

ground water in poorly drained 

areas

High concentrations of dissolved 

iron are typical in poorly oxygenated ground water on the Delmarva 

Peninsula. Concentrations of dissolved 

iron were greater than 300 μg/L, the 

Secondary Maximum Contaminant Level 

for drinking water (U.S. Environmental 

Protection Agency, 2002a), in 24 percent 

of the water samples from shallow 

wells in agricultural areas, 31 percent 

of the samples from wells completed 

in the parts of the surficial aquifer used 

for domestic supply, and 17 percent 

of samples from public-supply wells 

in the surficial aquifer in Delaware. 

Concentrations of dissolved iron in all 

wells were generally below 100 μg/L in 

water with concentrations of dissolved 

oxygen greater than 1 mg/L (fig. 27). 

Although dissolved iron is not a humanhealth concern, it affects the taste of 

water and can stain water fixtures at 

high concentrations, making water treatment desirable for domestic and public 

water use.

Dissolved iron is derived principally from the dissolution of naturally 

occurring iron minerals that coat sand 

particles; it also may be derived from 

the oxidation of pyrite in sediments 

containing organic matter of marine 

origin. Reducing conditions that cause 

the mobility of dissolved iron are commonly present in water that recharges 

the surficial aquifer through poorly 

drained settings with fine-grained 

sediments, such as forested wetlands. 

Reducing conditions also occur near 

underlying confining beds where water 

from deeper aquifers is discharging 

upward and in parts of the surficial 

aquifer with shallow semiconfining 

beds.

The highest concentrations of 

dissolved iron in shallow ground water 

on the Delmarva Peninsula were found 

in the south-central part of the peninsula, near the Pocomoke River (fig. 1). 

In this area, the surficial aquifer is 

confined or semiconfined by fine-grained 

sediments with high organic-matter 

content that causes reducing conditions 

in shallow ground water. Concentrations 

of dissolved iron in stream water also are 

high in this area compared to those in 

well-drained watersheds. In some areas, 

concentrations of dissolved iron are so 

high in the ground water discharging to 

streams that iron-rich sedimentary rocks 

are formed in the streambeds (fig. 28).

Figure 27. Concentrations of 

dissolved iron are often high 

in water from the surficial 

aquifer where dissolved 

oxygen is absent or is at very 

low levels.

Figure 28. Bog ore mined from the 

swamps in the Pocomoke River watershed was used to produce pig iron at the 

Nassawango Iron Furnace near Snow 

Hill, Maryland, between 1832 and 1850.

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

IRON, IN MICROGRAMS PER LITER

Shallow ground water in agricultural areas

Ground water used for domestic supply

Ground water used for public supply

100,000

1

10

100

1,000

10,000

2 4 6 10 0 8

Arsenic is common in ground 

water at low concentrations

Arsenic was detected in about 

one-half of the wells sampled in the 

surficial aquifer in agricultural areas 

and in areas used for domestic supply. Concentrations generally were 

low, exceeding the standard of 10 μg/L 

(U.S. Environmental Protection Agency, 

2002b) in only one shallow agricultural 

well in the coastal lowlands along the 

western shore of the peninsula. The 

major natural sources of arsenic in 

surficial-aquifer sediments are buried 

organic matter and associated iron minerals of marine origin. Poultry manure 

applied to agricultural fields is another 

potential source of arsenic in shallow 

ground water and surface water on the 

Delmarva Peninsula. (See inset on next 

page.) 

Arsenic is detected primarily as a 

dissolved ion under reducing conditions 

in ground water, conditions that typically also promote high concentrations 
Major Findings 21

of dissolved iron. These conditions exist 

in the south-central part of the peninsula 

near the Pocomoke River watershed. In 

this area, arsenic is detected in confined 

and semiconfined parts of the surficial 

aquifer where recharging ground water 

has passed through organic-rich marine 

sediments. The arsenic in this part of the 

surficial aquifer is believed to be primarily from natural sources in the overlying 

sediments.

Although arsenic occurs naturally, 

it is a widespread environmental contaminant and a growing concern in drinkingwater supplies. The American Cancer 

Additional Information

For more information on water 

quality in the surficial aquifer 

of the Delmarva Peninsula go to 

URL http://md.water.usgs.gov/

delmarva/.

Organic arsenic feed additives 

are used in poultry production for 

increasing weight gain, improving 

feed efficiency and pigmentation, 

and controlling bacterial and parasitic disease (U.S. Food and Drug 

Administration, 2000). Organic 

arsenic compounds, such as Roxarsone 

(3-nitro-4-hydroxyphenylarsonic acid), 

do not accumulate in poultry tissue or 

feathers (Aschbacher and Feil, 1991) 

but instead are rapidly excreted, resulting in elevated concentrations of arsenic 

in manure (Morrison, 1969). Poultry 

manure applied to agricultural fields 

introduces arsenic into the environment 

and may result in increased amounts 

of arsenic in stream water and ground 

water and uptake by plants.

 Results of a reconnaissance 

investigation in the Pocomoke River 

Basin (fig. 1) in an area with land 

Society recently recognized that arsenic 

causes cancer of the liver, lung, and skin 

(Heath and Fontham, 2001). In 1975, the 

U.S. Environmental Protection Agency 

established the maximum contaminant 

level for arsenic at 50 μg/L in drinking 

water; in 2001, the standard was lowered to 10 μg/L (U.S. Environmental 

Protection Agency, 2000), which also 

is the current provisional guide used by 

the World Health Organization (World 

Health Organization, 1999). Arsenic 

also is toxic to fish and many aquatic 

organisms (Canadian Council of 

Ministers of the Environment, 2002) 

Reconnaissance for Arsenic in a Poultry-Dominated Basin on the Delmarva Peninsula

Land application of poultry waste organic 

arsenic compounds may release arsenic 

into the environment and may result in 

increased arsenic in surface water and 

ground water and increased uptake by 

plants and aquatic organisms. 

and, if present in soil water, it may be 

taken up by plants—rendering them 

unsuitable for human or animal consumption (Marin and others, 1993; 

Krishnamurti and Naidu, 2002).

application of poultry manure indicate 

that some of the arsenic applied to 

agricultural fields is released into the 

environment on the Delmarva Peninsula. 

Highest concentrations of arsenic were 

measured in fresh manure; aged manure 

and soils contained lower concentrations 

because arsenical feed additives, such 

as Roxarsone, leach rapidly from the 

manure into water and are transformed 

into inorganic forms during composting 

(Garbarino and others, 2003). 

 Arsenic concentrations were 

elevated above background levels 

(1–2 μg/L) in stormwater runoff, 

soil pore water, and some samples of 

shallow ground water beneath agricultural fields (see table, this page). 

Concentrations were near background 

levels in most stream-water and groundwater samples from the Delmarva 

Peninsula (Welch and others, 2000; 

Canadian Council of Ministers of 

the Environment, 2002). Arsenic 

was detected in water samples from 

the deeper aquifer, which is used 

for domestic supplies in this area, 

at concentrations less than the current Federal drinking-water standard 

of 10 μg/L (U.S. Environmental 

Protection Agency, 2000). Because 

the deeper aquifer is confined in this 

area, the arsenic present is believed to 

be from natural sediment sources.

Solid material

Fresh poultry manure 27

Aged poultry manure 1.0 to 2.0 

Soil 1.0 to 2.0 

Streambed sediment 0.3 to 17 

Ground water Total dissolved

 arsenic (μg/L)

Deeper ground water Less than 1.0 to 7.6

Shallow ground water Less than 0.1 to 23 

 Pore water, shallow core in

agricultural field 6.8 to 29 

Filtered surface water 

(dissolved)

Base flow 0.5 to 1.6 

Stormflow 0.5 to 10.4 

Unfiltered surface water

(particulate + dissolved)

Total arsenic

 (μg/L)

Stormflow 1.16 to 9.4 

Total arsenic

(mg/kg)

Total dissolved

 arsenic (μg/L)

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Study Unit Design

Water quality in the surficial 

(unconfined) aquifer system and the 

streams that receive most of their base 

flow from shallow ground-water discharge was the main focus of this study. 

The current Delmarva Peninsula study 

was a continuation of a pilot NAWQA 

project conducted during 1987–91. 

Some of the pilot project’s ground-water 

networks were resampled for this project 

as part of the study of trends in groundwater quality. The effects of agriculture 

on water quality was targeted for study 

because agriculture is the predominant 

land use on the Delmarva Peninsula. 

Results from this study are compared to 

those in other agricultural areas around 

the Nation.

Stream Chemistry

Two basic fixed sites in watersheds 

with predominantly agricultural land use, 

but in different hydrogeologic settings, 

were sampled frequently to determine 

the occurrence and seasonal variability of 

nutrients and pesticides in surface water.

Ground-Water Chemistry

An aquifer survey was done to 

assess overall water quality in the part 

of the aquifer that is used as a source 

the two basic fixed sites in collaboration with a study of fish health being 

done as part of the USGS Chesapeake 

Bay studies. A synoptic study of water 

chemistry during low streamflow in 

small watersheds draining mixed land 

uses was done in collaboration with the 

U.S. Environmental Protection Agency. 

Stream sites for this study were selected 

by means of an unequal probability 

design. With this design, chemical 

results from the 23 headwater streams 

sampled on the Delmarva Peninsula 

were used to provide an unbiased estimate of regional stream conditions 

(Ator and others, 2003).

for domestic drinking water. A land-use 

effects study in agricultural settings was 

done in the shallow part of the surficial 

aquifer underlying areas of agricultural 

land use. Public-supply wells screened 

in the surficial aquifer were sampled 

as part of a collaborative source-water 

assessment project with the State 

of Delaware. The apparent age of 

ground water increased as well depth 

increased in these three networks. Four 

agricultural flow-path studies examined 

the transport and fate of nutrients in 

water of the surficial aquifer on a local 

scale.

Special Studies

A reconnaissance of arsenic in 

ground water, stream water, streambed 

sediment, soil, and aquifer material 

was done in the watershed upstream 

from the Pocomoke River fixed site. 

Pesticide concentrations were measured 

in estuarine waters downstream from 

Ground water in agricultural areas

Ground water used for domestic supply

Ground water used for public supply

Maximum

Median

Minimum

29 Number of samples

28

30

29

29

29

29

APPARENT AGE

DEPTH, IN FEET BELOW LAND SURFACE

2000

1990

1980

1970

140

120

110

130

100

80

70

60

50

40

30

20

10

0

90

EXPLANATION

22 

39°

76°

76°

38° 38°

STREAM-SAMPLING SITES

KILOMETERS

MILES

10

EXPLANATION

Sampling sites

Basic Fixed sites

Small stream synoptic

Estuarine survey

Pocomoke arsenic study

u

s

10

10 20

KILOMETERS

MILES

s

e

s

Urban

Agricultural

Forest

Water

Wetland

Land use

Sampling sites

Aquifer survey

Land-use effectsagriculture

Flow-path studies

Aquifer survey−Delaware

Note: Most "aquifer survey"

wells are collocated with "landuse effects−agriculture" wells

and do not show on this map.
Study Unit Design 23

Summary of data collection in the Delmarva Peninsula

Study

component What data are collected and why Types of sites sampled Number of sites sampled

Sampling frequency 

and period

Stream chemistry

Basic fixed 

sites—

agriculture

Streamflow, pesticides, pesticide degradates, nutrients, major ions, suspended 

sediment, specific conductance, pH, and 

dissolved oxygen; to describe occurrence and concentrations and to evaluate 

seasonal, temporal, and flow-related 

variations.

Streams that drain primarily 

agricultural watersheds.

 2 Monthly, 

September 1999–

September 2001

Ground-water chemistry

Aquifer survey 

(surfical aquifer 

used for domestic supply)

Pesticides, pesticide degradates, 

nutrients, major ions, trace metals, 

radionuclides, bacteria, specific conductance, turbidity, pH, and dissolved 

oxygen; to describe water chemistry 

in the surficial unconfined aquifer on 

the peninsula.

Monitoring (16) or domestic (13) 

wells, selected using a random 

approach (Scott, 1990). Median 

well depth, 45 ft. 

 29 Once, in 2001

Aquifer survey—

Delaware

Pesticides, pesticide degradates, volatile 

organic compounds, nutrients, major 

ions, radium, radon, specific conductance, turbidity, pH, and dissolved 

oxygen; to describe water chemistry in 

the surficial unconfined aquifer where 

used for public supply in Delaware.

Public drinking-water supply wells 

(sample collected prior to any 

treatment) in surficial aquifer 

in Delaware, selected using a 

random approach (Scott, 1990). 

Median well depth, 80 ft.

 30 Once, in 2000

Land-use effects—

agriculture 

(shallow wells 

underlying 

agriculture)

Pesticides, pesticide degradates, nutrients, major ions, specific conductance, 

turbidity, pH, and dissolved oxygen; 

to describe water chemistry in the 

surficial unconfined aquifer in agricultural areas of the peninsula.

Shallow monitoring (27) and domestic (2) wells, located in areas 

with agricultural land using a 

random approach (Scott, 1990). 

Median well depth, 22 ft.

 29 Once, in 2001, and 

December 2002–

January 2003

Flow-path studies Pesticides, pesticide degradates, nutrients, major ions, specific conductance, 

pH, and dissolved oxygen; to document the movement of chemicals into 

and through the surficial unconfined 

aquifer in agricultural areas of the 

peninsula.

Shallow monitoring wells and 

streams in selected small 

watersheds. Various depths.

4

watersheds

Once, in 1999

Special studies

Pocomoke arsenic 

study

Arsenic compounds; to determine the 

fate and transport of arsenic within the 

upper Pocomoke watershed.

Small streams, wells, soils, soil 

pore water, and poultry manure 

from several locations within 

the watershed.

1

watershed

Once, in 1999

Estuarine survey Pesticides, pesticide degradates, nutrients, specific conductance, turbidity, 

pH, and dissolved oxygen; to evaluate 

the transport of chemicals from flowing streams to tidal estuaries and to 

support ongoing studies of fish health.

Tidal estuaries, downstream from 

basic fixed sites.

 2 Quarterly, 2001 

and 2002

Small stream 

synoptic

Streamflow, pesticides, pesticide 

degradates, nutrients, major ions, specific conductance, pH, and dissolved 

oxygen; to describe the occurrence 

of concentrations in ground water 

discharging to small streams.

Small, first-order streams, randomly selected to represent various 

land uses (Pitchford and others, 

2000; Ator and others, 2003).

 23 Once, spring 2000, 

during base-flow 

conditions
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24
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National Atmospheric Deposition Program, 2003b, Estimated 

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Algae—Chlorophyll-bearing nonvascular, primarily aquatic 

species that have no true roots, stems, or leaves; most algae are 

microscopic, but some species can be as large as vascular plants. 

Aquifer—A water-bearing layer of soil, sand, gravel, or rock 

that will yield usable quantities of water to a well. 

Atmospheric deposition—The transfer of substances from 

the air to the surface of the Earth, either in wet form (rain, fog, 

snow, dew, frost, hail) or dry form (gases, aerosols, particles).

Base flow—Sustained, low flow in a stream; ground-water 

discharge is the source of base flow in most streams.

Basic fixed site—Site on a stream at which streamflow is 

measured and samples are collected for temperature, salinity, 

suspended sediment, major ions and metals, nutrients, and 

organic carbon.

Confined aquifer (artesian aquifer)—An aquifer that is 

completely filled with water under pressure and that is overlain 

by a fine-grained material that restricts the movement of water. 

Degradate (degradation product)—Compounds resulting 

from transformation of an organic substance through chemical, 

photochemical, and(or) biochemical reactions. 

Denitrification—A process by which oxidized forms of 

nitrogen such as nitrate (NO3

–

) are reduced to form nitrogen 

oxides or free nitrogen: commonly brought about by the action 

of denitrifying bacteria and usually resulting in the escape of 

nitrogen to the air.

Discharge—Rate of fluid flow or a constituent of flow passing 

a given point at a given moment in time, expressed as volume 

or mass per unit of time.

Drinking-water standard or guideline—A threshold 

concentration in a public drinking-water supply, designed 

to protect human health. As defined here, standards are 

U.S. Environmental Protection Agency regulations that specify 

the maximum contamination levels for public water systems 

required to protect the public welfare; guidelines have no regulatory status and are issued in an advisory capacity. 

Eutrophication—The process by which water becomes 

enriched with plant nutrients, most commonly phosphorus and 

nitrogen. Eutrophication can cause excessive growth of aquatic 

plant and algae in water bodies. Decay of this biomass can 

result in low concentrations of dissolved oxygen in the water 

column.

Ground water—In general, any water that exists beneath 

the land surface, but more commonly applied to water in fully 

saturated soils and geologic formations.

Headwaters—The source and upper part of a stream.

Herbicide—A chemical or other agent applied for the purpose 

of killing undesirable plants. See also Pesticide.

Insecticide—A substance or mixture of substances intended to 

destroy or repel insects. See also Pesticide. 

Median—The middle or central value in a distribution of data 

ranked in order of magnitude. The median also is known as the 

50th percentile. 

Micrograms per liter (μg/L)—A unit expressing the concentration of chemical constituents in solution as mass (micrograms) of solute per volume (liter) of water; equivalent to one 

part per billion in most stream water and ground water. One 

thousand micrograms per liter equals 1 milligram per liter. 

Milligrams per liter (mg/L)—A unit expressing the concentration of chemical constituents in solution as mass (milligrams) of solute per unit volume (liter) of water; equivalent to 

one part per million in most stream water and ground water. 

Nutrient—Element or compound essential for animal and 

plant growth. Common nutrients in fertilizer include nitrogen, 

phosphorus, and potassium. 

Pesticide—A chemical applied to crops, rights-of-way, 

lawns, or residences to control weeds, insects, fungi, nematodes, rodents, or other “pests.” 

Picocurie (pCi)—One trillionth (10–12) of the amount of radioactivity represented by a curie (Ci). A curie is the amount of 

radioactivity that yields 3.7 × 1010 radioactive disintegrations 

per second (dps). A picocurie yields 2.22 disintegrations per 

minute (dpm) or 0.037 dps. 

Recharge—Water that infiltrates the ground and reaches the 

saturated zone. 

Riparian—Pertaining to or located on the bank of a body of 

water, especially a stream. 

Runoff—Excess rainwater or snowmelt that is transported to 

streams by overland flow, tile drains, or ground water. 

Surface water—An open body of water, such as a lake, river, 

or stream. 

Surficial aquifer—The shallowest aquifer whose upper surface is the water table; the aquifer on the Delmarva Peninsula 

containing unconfined ground water.

Upgradient—Of or pertaining to the place(s) from which 

ground water originated or traveled through before reaching a 

given point in the aquifer.

Volatile organic compounds (VOCs)—Organic chemicals that 

have a high vapor pressure relative to their water solubility and 

which evaporate readily at normal pressures and temperatures. 

VOCs include components of gasoline, fuel oils, and lubricants, 

as well as organic solvents, fumigants, some inert ingredients in 

pesticides, and some byproducts of chlorine disinfection. 

Watershed—The portion of the surface of the Earth that contributes water to a stream through overland runoff, including 

tributaries and impoundments.

Water table—The point below the land surface at which 

ground water is first encountered and below which the earth 

is saturated. Depth to the water table varies widely across the 

country. 

Wetlands—Ecosystems whose soil is saturated for long periods seasonally or continuously, including marshes, swamps, 

and ephemeral ponds.

Glossary

26
Appendix—Water-Quality Data from the Delmarva Peninsula in 

a National Context

27

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Appendix 29

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Coordination with agencies and organizations in the Delmarva Peninsula was integral to the success 

of this water-quality assessment. We thank those agencies and organizations that cooperated with 

data-collection efforts, contributed information, and participated on the liaison committee.

We also thank the following individuals for contributing to this effort.

The numerous property and business owners who allowed us access to their wells and streams for sampling.

Deborah Bringman, George Zynjuk, John Jastram, and David Hudson of the U.S. Geological Survey who provided data 

collection and technical support.

Personnel from the Delaware Geological Survey and U.S. Geological Survey Geologic Division for well-drilling services.

John Batista, U.S. Geological Survey Volunteer for Science for photography of field sites.

Gerhart Reidel, Academy of Natural Sciences, and John Garbarino, U.S. Geological Survey National Research Program for 

arsenic analysis.

Report reviewers and preparers including: William Susan and Richard Fleming, private citizens; John Barndt, Delaware 

Department of National Resources and Environmental Control, and James Stark; U.S. Geological Survey, colleague 

reviewers.

Federal Agencies

U.S. Department of Agriculture

 Natural Resources Conservation Service

U.S. Environmental Protection Agency

 National Exposure Research Laboratory

 Landscape Ecology Branch

 Office of Research and Development

 Mid-Atlantic Integrated Assessment

 Region III

State and Local Agencies

Accomac/Northhampton Planning District

Delaware Department of Agriculture

Delaware Department of Natural Resources and Environmental Control

Delaware Department of Health and Social Services

Delaware Geological Survey

Maryland Department of the Environment

Maryland Department of Natural Resources

Virginia Department of Environmental Quality

Virginia Department of Health

Universities

University of Delaware

University of Maryland
Denver and others— Water Quality in the Delmarva Peninsula 

U.S. Geological Survey Circular 1228

National Water-Quality Assessment (NAWQA) Program

Delmarva Peninsula

NAWQA

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